gem development environment

[coldnebo]

EDIT: Well, I should have read the readme steps I suppose... 😄

In any case, the PR streamlines this from:

$ bundle install
$ cd ext/mimemagic/
$ bundle exec rake
$ popd

$ bundle exec rake test

to

$ bundle install
$ bundle exec rake

---

I noticed that the gem development environment is missing some files.

$ rake test

File does not exist: mimemagic/path

rake aborted!

I see that the ext rakefile has to run first and am working on a PR to fix.