Recognize Makefiles in deeper subdirectories
Ignore .travis.yml by default (#39)
Correctly handle a relative --python path (#36)
Warn about leading/trailing slashes in MANIFEST.in (#37)
More terse output by default; -v/--verbose option

I'm not entirely happy with the output API: error()/warning()/info() are
always shown, while info_begin()/info_confinue()/info_end() are shown
only in verbose mode.