Support Rails' multiple database feature

[derekprior]

For some time now, Rails has offered multiple database support. Scenic, however, operated with the assumption that there is one database, hanging its functionality off the singleton, Scenic.database. We should make the changes necessary to support multiple databases, which may have the added benefit of easily offering view path configurability as has been asked for by a number of folks.

I'm currently messing with multiple databases in a sample app and will use this issue to enumerate the changes I think we need to make.

• [ ] Teach our generators about the --database option
• [ ] Use context available within a migration to execute DDL in the correct database
• [ ] Update methods accessed from models at runtime to respect the database used in the model

[leoplct]

In the meanwhile, what workaround could I use to run Scenic on my secondary database?