Merge pull request #16 from comstud/mysqldb-dev

Created bw_usage_cache MixIn and moved update() code.
Merge pull request #17 from comstud/mysqldb-dev

Mysqldb dev
Merge branch 'bp/db-mysqldb-impl' of github.com:comstud/nova into bp/db-mysqldb-impl
more bug fixes
Merge branch 'mysqldb-dev' into bp/db-mysqldb-impl
removed unused methods from connection
pep8 fixes
more pep8