Remove unused third-party-tools folder.
Merge pull request #710 from Renelvon/no_3rd_party

Remove unused third-party-tools folder.
Factor GUIDMixin definition in a new file.

* Abstract initialization of GUIDMixin inside
  initialization of CryptoPeerConnection
* Coerce string to GUIDMixin when adding contact.
Support unicode GUIDs.

* Add a deprecation comment for future comparisons
  against subclasses of basestring.
Test GUIDMixin.

* Also test CryptoPeerConnection subclassing.
Update documentation.