Fixes #97, error following 'DROP TABLE'.

Following 'DROP TABLE', metadata.py was raising KeyError in line 162,
which resulted in cluster conn being terminated. Resolved this with
checking if the key (the name of the dropped table) exists.
Use pop instead of del if exists
Update changelog

Fixes issues #97 and #98
Add debug logging around trace fetching
Account for CASSANDRA-6942 in tracing tests

Fixes PYTHON-57
Correct catch Empty exception on connection id queue

Broken in master, but not in any release
Debian packaging
Merge pull request #101 from thepaul/debian

Debian packaging
Add methods for concurrent execution

Fixes #7
Use absolute imports in cassandra.cluster
Make test_concurrent compatible with master
Update changelog
Docs for cassandra.concurrent