Implement further changes required for virtualenv

don't assert() if python errors inside of a CFGVisitor's RAII scope

INSTALLING.md: add instructions on getting gdb pretty-printers to work

- make sure that, except for a few special cases, any type which
  has instance attributes gets a __dict__ descriptor
- remove instance attributes from generator and builtin_function_or_method
  objects

- oldstyle classes get a __dict__
- object().__hash__()
- old style classes __getattribute__
- str.splitlines()
- and file(unicode, unicode)
- start_new_thread takes a third argument
- set.union
- instancemethod __getattr__

Fixing makefile link

fixing makefile link

Implement set comprehensions, fix dict comprehension scope

Document that we need libssl-dev

We were already doing this for CAPI classes.
Only slightly trickier due to bootstrapping issues.

object.__reduce__, __reduce_ex__, and some friends

1) because we haven't seen the types multiple times yet,
so creating a new function specialization could be wasteful, and
2) the interpreter can't make use of the type specialization.

Specializing at the interpreter does let us start counting how
many times each specialization is used, and the function entry
counts are the only way we have right now of telling that a
particular specialization should be moved to a higher tier.
But I don't think we should do that using this mechanism.

ex:

for i in xrange(5):
    pass
else:
    return 2
"unreachable here!"

We didn't prune the "unreachable here" line, which caused
some issues.  I guess we could also support compiling
trivially-unreachable blocks.