Better test coverage, make strategy methods public

Closes #7. This change makes Strategy#authenticate and Strategy#receive
public by default, and refactors test coverage to execute #receive rather
than instantiating the receiver and then testing the result of #message,
which is why the tests didn't catch the protected method bug in the first
place.
Normalize attachments from request params.

Closes #6. Rails handles uploaded files differently from Rack.
Specifically, Rack returns a hash containing file data, while Rails
returns an ActionDispatch::Http::UploadedFile object.
Rely on headers for parsing to/from. /cc #5
Test multiple mailgun recipients /cc #5
Fall back to full body if stripped body is empty
Version 0.1.5
Release 0.1.5
Merge branch 'master' of https://github.com/honeybadger-io/incoming