fix interface to megam maxent classifier!

Small change, but it makes megam work with Pythons 2 and 3.

- Instead of writing a gzipped training file, just write a regular text file. (megam is about to decompress it anyway) -- this avoids the hassle of writing binary files in the gzip library, which doesn't like unicode strings.
- Make sure, when we get the output of megam, that we turn it back into a standard string for the current Python.