fixed a bug HashType when converting a value; the fix just tests the value to ensure it is a hexadecimal string rather than converting the value to the a long integer.  Without this fix, the final assertion in test_md5_type in test_serialize.py fails
Merge branch 'upstream/master'
Merging in a change to the MD5Type from Ryan Olson.

Merge branch 'master' of https://github.com/ryanolson/schematics into ryanolson