Give adapters control over their destiny

Adapters now declare whether they are search adapters, item adapters, whether they are a search type or cite type, whether they are syncable, etc. This eliminates `subscriptions.yml`.

You may also change the path to the adapters with an environment variable, allowing you to have a directory full of your own, custom adapters.

`adapter_info` in `environment.rb` does the heavy-lifting. All the other methods live off of it - except for the Rake tasks, which need to figure stuff out before loading the environment.