Added code to handle loading libexempi on macports.  #6

macports will install libexempi.dylib in /opt/local/lib, which ctypes
may not normally find on its own.
Merge branch 'issue6' into devel
Added test_skip to verify usage of XMPMeta skip method.  Closes #4
Merge branch 'issue4' into devel
Merge pull request #31 from python-xmp-toolkit/devel

Devel to merge into master