Implement PDF processing. Includes:

* Numerous spec improvements
* Refactor the existing processor classes to have more logical names
* Create a way to dispatch to different processors based on mime types
* Fix the carrierwave config
* Remove unused minitests

Todo:

- [x] Implement the logic that actually parses PDF content
Use 2 spaces for indentation in an array, relative to the start of the line where the left bracket is.