Move 3rd party libraries to deps/ directory.

Move gevent and greentest to src/ directory. This fixes tox.

At this point the builds should all work the same. Lets check.

This makes the builds faster due to smaller .c output to merge. Plus all
the improvements in Cython itself.

*NOTE* Not taking advantage of the new @property supporty yet, in case
 there are any changes to the .ppyx file we need to backport to 1.1.

Move cdef and source from Python strings to .c files that can be parsed and validated by a real C compiler.

reorganize changelog entries for 1.2a1 to group related entries. it was getting hard to follow. [skip ci]

Actually it wasn't quite as dire as described there. Instead, there were
just extra copies of each chunk of data the application yielded.

This commit removes the copies, sending the data directly to the socket.
This should be most noticeable on larger chunks.

Also re-organize the call to close() the app iter closer to where the
app iter gets assigned, making it easier to spot. Add a test for
this---turns out the validator was closing the iterator for us!

I found one genuine cycle in callback functions. The rest I can't find
any legit leak, so for now I'm disabling that part of the test on the TPE.

It now works as expected when both patched and not patched. It is
cooperative with greenlets in both cases. Test this.

This can be helpful when threading is patched.

Ref #786.

I can't reproduce with Python 2.7 locally, and the error makes no
sense (because we're writing the banner to a *brand bloody new* file,
guaranteed not to exist). So this is a shot in the dark.

See
https://ci.appveyor.com/project/denik/gevent/build/1.0.568/job/p44icqoi91a72e08.

Failed to rename 'c:\users\appveyor\appdata\local\temp\1\tmp_j5icn\gevent\corecext.pyx.tmp.2832' to 'c:\users\appveyor\appdata\local\temp\1\tmp_j5icn\gevent\corecext.pyx
Exception in thread Thread-1:
Traceback (most recent call last):
  File "C:\Python27\lib\threading.py", line 801, in __bootstrap_inner
    self.run()
  File "util\cythonpp.py", line 31, in run
    self.value = target(*args)
  File "util\cythonpp.py", line 432, in _run_cython_on_file
    atomic_write(unique_pyx_filename, py_banner + value)
  File "util\cythonpp.py", line 902, in atomic_write
    os.rename(tmpname, filename)
WindowsError: [Error 32] The process cannot access the file because it is being used by another process

This permits some more optimizations and speeds up the build another 20%
or so, in addition to being hopefully easier to follow.

On my machine, this cuts the build time by a fourth to a third.

Not ready to merge, submitting for testing on CI (esp Windows).

Cython calls are threaded because they release the GIL.

The most expensive part was merging results, which holds the GIL, so use
multiprocessing for that. Pre-combine the identical results that we
expect to get for two sets of defines to reduce the number of merges.

Change internal data structures to be immutable to make debugging this
easier (and substantially faster when pickling through multiprocessing.)

Don't subclass dict for environ by default, but leave that option
available. When printing, make sure that we have a secure environ.
Unfortunately, this is relatively fragile but it's the only way to not
break WebOb/Pyramid.