Look for custom processors in `lib/paperclip`

... instead of `lib` of `lib/paperclip_processors`.
Fixes thoughtbot/paperclip#1534
Re-add lib/paperclip_processors for custom processors.

Also, add integration tests for custom processor definition and unit
test for Paperclip.load_processor (thoughtbot/paperclip#1535).
Reflect custom processor changes on README.md.

(thoughtbot/paperclip#1535)