Return an iterator, not an iterable.
Fix up broken/incomplete parabench.
Tidy up module init/initial TLS callbacks.

This adds a `callback_tls' global which is a Python-level
threading.local to be used for storing callback state. This avoids the
need to depend on OS-specific APIs.
Merge branch 'master' of github.com:dw/py-lmdb
Kill TLS, complete readers() and check_readers() for cpython.

TLS was congnitive distortion due to how the cffi implementation must be
done. Rename cffi readers_check() to check_readers(), to match the C
API.
Remove needless state struct.
LMDB_0.9.7 @ git://gitorious.org/mdb/mdb.git
Bump version for release.