Make scales, blist optional dependencies

Fixes #114
Prefer lz4 compression, allow specifying compression type

This is related to #1
Add blist to tox dependencies
Don't make blist a dep for pypy tox test
Add comment about compression ordering
Add blist to test-requirements.txt
Merge branch 'master' into 2.0

Conflicts:
	cassandra/connection.py
	requirements.txt
	setup.py
	tests/unit/test_connection.py
	tox.ini