allow use of multiple database configs

This lets Combustion be initialized for a database configuration other
than 'test'. The name of the config block to use is passed as an
optional hash argument to Combustion.initialize!, like so:

    Combustion.initialize! :active_record, :database => 'other'

--HG--
extra : rebase_source : 92dadc2e06a947609102759f252992721e19a7c1
extra : histedit_source : 6f35f546af689b5771269bc6ed6d930c9b045c3e
load db/schema_<database>.rb if it exists

--HG--
extra : rebase_source : a8fbb334d4fda9e4e6ada3cb1243ff04223b5d60
extra : histedit_source : 07352cd1a7554de4808b1564cb7ec275cb83e3a5
database: allow passing db config name as symbol

--HG--
extra : rebase_source : d7f2330437658a19644a1f5d09841bcd12140f5e
extra : histedit_source : 3d278d805233f6631114bfe202655dc5f0b6391f