Deploy development documentation from Travis [not ready to merge]

This is as much to get some input and see if this makes any sense as a final pull request.

The idea is to upload the documentation build by Travis on the master branch to a git repository on the gh-pages branch. For security this is done in a separate repository from the main code. Travis stores an encrypted private deployment key which can only be used to upload to this repository. According to the travis docs this key can only be decrypted on a main branch and not a PR to protect against leaks. 

See http://docs.travis-ci.com/user/deployment/custom/ for the Travis documentation 

I imaging that this should be linked from the main list of version on matplotlib.org For this to work as intended it will also require modification of versions.html in the main repository to use absolute rather than relative links and point to this version. 

I originally tried using a submodule but that will require modifying the main repo when a new branch is pushed
to link it to the tip of the dev repository.

The one that I am currently building can be seen at http://jenshnielsen.github.io/mplddocs/index.html (Builds every time that I push against the master branch of my mpl fork)

And my fork of the main documentation at http://jenshnielsen.github.io/matplotlibdocs/ with links to the development docs

What do you think? Does it make any sense to upload the development documentation? 
I think if you do `git checkout --orphan gh-pages` than this will create an orphaned commit, meaning it has no parent.  That way, this repository won't grow to an an unwieldy size.  I don't think there's any need to keep history here.