Merge pull request #5 from RJacksonm1/master

Track whether a string is a key; Fixes #4.
Catch all exceptions in initial vdf bytearray conversion

It seems that python2 will throw an encode error when something is
already decoded, but 3 will not throw at all in some case or will
just throw a type error. Since the cause and result are the same
it may as well just catch globally
Add vdf serialization test fixture

and with it, a testcase failure on that pesky mixed vdf block.
Fixed vdf data after subnodes being ignored because of key-tracking code. Fixes #6

Adjusted test to include data after a subnode.