schema and database awareness

[jenseng]

as indicated in the readme, immigrant might detect "missing" keys if you use multiple databases or schemas. those "missing" keys are actually:

1. keys that already exist (e.g. between two tables in a different db),
2. keys that cannot exist (from one database to another), or
3. keys that should probably not exist (from one schema to another, e.g. if you're using the apartment gem ... while they could be created, it all falls apart once you move beyond a single db ... see point 2.)

immigrant should:

1. ask each model's connection for its foreign keys, rather than ActiveRecord::Base.connection ... this partially addresses the first problem
2. ignore associations across databases
3. for tables within the same database, ignore associations across schemas (for apartment+postgres at least, we can tell this from the table_name's prefix ... "public." vs. ""). might need to do a little research to make sure this is done in a sharding-agnostic way (if possible)