Clean up module structure a bit

Move code from libcoucbase.py to connection.py
Fixup some doc references

- old libcouchbase.py name still available
- Moved over common types and constants to their own modules

Change-Id: Ife557ad40b0adad10da65d1bc2886ca155c59c95
Reviewed-on: http://review.couchbase.org/26824
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>