MASC reader

Hi,

here is the pull request for the MASC reader.

1) A CorpusReader class parallel to the others. Needs "graf-python" package, which is checked at runtime when the class is used. A message is printed when the package is not in installed.

2) Upate to the corpus.doctest and tox.ini. I added "graf-python" to tox in oder to be able to run the tests.

Hope everything is fine.

The current URL to the corpus files is:

http://www.anc.org/downloads/MASC-2.0.0.zip

Best,
Peter