added write_gml() in zen.io.gml
update gml.py to match internal
make zen.io.gml.read() expect utf-8-compatible encoding
add gml tokenizer
add gml tokenizer unit tests

update Tokenizer to raise exception when eof encountered while
expecting value token.
Add gml_interpreter. Not tested yet
add tests for gml tokenizer, interpreter, and reader
add ability to read/write bipartite graphs in gml
add ability to write gml graphs with tuple nobj
merged official external release into my master
remove completed inline TODOs
Merge pull request #20 from enewe101/master

improve gml read/write facilities for zen