Update corpus metadata files

[alexrudnick]

Check that the license and other information in the corpus index files is correct.

Migrated from http://code.google.com/p/nltk/issues/detail?id=96

[fcbond]

It would be nice to have a standard interface to access the license and citation interface, as we do for the readme.

I suggest adding to the API something like:

   def license(self):
        """
        Return the contents of the corpus LICENSE file, if it exists.
        """
        if os.path.exist(self._root.join("LICENSE")):
            return self.open("LICENSE").read()
        else:
            return "No LICENSE found for this corpus (maybe check the README)"

And the same for '''citation' which would look for "citation.bib".

Then we need to add the LICENSE and citation info for each corpus. I will do it for the open multilingual wordnet and wordnet.

[stevenbird]

Good idea. I've added this, but without the conditional, since I think it's fine let Python generate an error message. I see that there are many corpora containing README.txt instead of README, so that also needs to be standardized.