Repository organization - split up front end and back end

[aalpern]

This will make it easier to implement #467.

The front-end and back-end source trees are intermingled right now, at the root of the repository. They're largely independent, however, and can be split into separate source trees to keep the separation clean, and facilitate splitting the front-end code into more reusable modules. We can use Github's scripts to rule them all pattern to coordinate running the Javascript and Python build steps with a single command.

+ tessera-frontend
  - Gruntfile.js
  - package.json
  - src/ts/etc...
+ tessera-server
  - setup.py
  - tasks.py
  - requirements.txt
  - tessera/etc...