Switch MAC addresses to hex strings rather than 2 numbers

T think this makes life a lot easier for the user, splitting MAC
addresses is not something they should have to worry about.

I also believe this fixes unsubscribe, which didn't look correct.
Fix some memory leaks and allow sending packets with embedded nulls

Apparently Utf8Value doesn't mess with null bytes like AsciiValue does.
Merge pull request #6 from helium/adt-hexstring-mac

Switch MAC addresses to hex strings rather than 2 numbers