Script to distribute a source package and dependencies.

push_package.py grabs performs the following:
* Parses a set of dependencies for a package from config.json.
* Grabs git branches from remotes associated with
  the package and dependencies (derived from checked out
  source trees).  Branch is retrieved from the remote of each
  project making sure no local changes are picked up by the
  process.
* Stages everything in a temporary directory.
* Pushes the package and dependencies to remote branches specified
  by config.json.
* Creates / updates a branch that contains the source package with
  its dependencies as submodules under the "dependencies" directory
  pushing the results to a remote branch.
* Updates documentation using docs/generate_docs.py, grabs the HTML
  output, stages the HTML in its' own branch ready to be pushed
  to a gh-pages branch.

Tested:
See https://github.com/google/fplutil
Also, tested with some local git repositories and a modified config.json
that pushes to these repos.

Bug: 18226263
Change-Id: Ia46b07a0b5b7afd53656e84e66f0104d35e66d89
Merge "Script to distribute a source package and dependencies." into ub-games-master
Cleaned up readme for github and added download links to landing page.

Tested:
Built the doxygen docs and verified it looks ok and all links work.
Bug: 18314286

Change-Id: Id4db555dbd9ebeea3239d8e4aacea158d17e8e97
Merge internal branch into master