Merge pull request #15 from comstud/mysqldb-dev

fix bugs with columns_to_join in instance_get_all
Mixin loading turns FooBar into foo_bar for the module name.
Merge branch 'bp/db-mysqldb-impl' of github.com:comstud/nova into bp/db-mysqldb-impl
Created bw_usage_cache MixIn and moved update() code.