Patch/Subset Code is now on the W3C Repo

Code is here:
https://github.com/w3c/PFE-analysis/tree/master/patch_subset

This provides an implementation of the Patch/Subset style of PFE as
specified in this design doc:
https://github.com/w3c/PFE-analysis/tree/master/patch_subset

I've included both a client and server implementation. Currently only the
very basic workflow of rebase and patch are supported. More advanced
features such as:

   - Codepoint remapping/re-indexing
   <https://docs.google.com/document/d/1DJ6VkUEZS2kvYZemIoX4fjCgqFXAtlRjpMlkOvblSts/edit#heading=h.z904uwbotomt>
   - Exceptional case handling
   <https://docs.google.com/document/d/1DJ6VkUEZS2kvYZemIoX4fjCgqFXAtlRjpMlkOvblSts/edit#heading=h.3tdo3ji8rnn5>

Are not yet implemented. Additionally no binary diff implementations are
currently included. I'm currently working on getting code for those
published.