Support different dialects in the Genome initializer
Support different dialects in the Genome initializer

Prior to this patch, Genome defaulted to SQLalchemy dialect defaults when connecting to a database: this meant that MySQLdb was used.
MySQLdb has no Python 3 support, and other alternatives are available, so as a first step this artificial limitation was removed.

Fixes issue #13.

brentp/cruzdb,brentp/cruzdb