Enable Travis CI builds
Add Rake test task (from Bundler template)
Rename test file according to Test::Unit conventions
Compile the C extension before testing

Compile with rake-compiler.
Make rake-compiler easily installable by `bundle install`.
Ignore all build artifacts (.gitignore from Bundler template).
Remove test/unit monkeypatch that test output really noisy
Correct test path to IMAGES_DIR

Fixes test failures of the type:

Magick::ImageMagickError: unable to open image
`../doc/ex/images/Button_0.gif':  @ error/blob.c/OpenBlob/2587