Fixed: ZODB.Connection.TransactionMetaData didn't custom data

  storage that some storages rely on.

sigh

transaction user and description: text above, bytes below

Also added _s to _transaction_meta_data_text_variables and
_text_transaction_info to mark them as private.

Conflicts:
	CHANGES.rst

WRT travis and winbot.

Conflicts:
	doc/guide/transactions-and-threading.rst

for user_name and description

Cleaned up check_checkCurrentSerialInTransaction

Weird, I thought I'd tested the transaction changes against ZODB. :/

- Use assertEqual rather than assertTrue for better error messages.

- Fixed a typo that caused an assertion to be missed.

Fix a few typos in documentation

Add a Gitter chat badge to README.rst

Remove obsolete code

They haven't been used in a long time.

2.6 is unsupported. 2.6 is long unsupported.

Consistently spell the version checks that remain as PY2 or PY3 to make
them slightly easier to find.