Support ecto prefixes

[michalmuskala]

The prefixes should change the database, once it's possible from the driver.

They should be supported in all queries and in migrations: elixir-ecto/ecto#1004

[Xorelock]

mongo_ecto/lib/mongo_ecto/connection.ex:45: (on the ecto-2 branch, using ericmj/mongodb master)

-opts = [projection: projection, sort: query.order] ++ query.opts ++ opts
+opts = [projection: projection, sort: query.order, database: query.database] ++ query.opts ++ opts

haven't figured out why it's query.database and not query.prefix, but that might be an implementation detail of NormalizedQuery... ?

This was a good excuse to learn elixir debugging tools. hope this helps!