Publishing built package

[hadley]

It would be vey useful if we had a recommended process for publishing the built package bundle on success. I don't think this necessarily means any extra code for r-travis, but it does need some discussion about conventions. Ideally, we'd end up with a standard way of publishing packages that me add a function to devtools which made getting a binary package from travis easy: install_travis("hadley/devtools"), install_travis("hadley/devtools@v1.4.1") etc.

Things to think about:

• How should we doctor the DESCRIPTION to make it clear that travis built the file?
• Where should we store the files? S3? Github pages? Github releases? It's probably sufficient to store only the most recent build for each branch
• Once travis support windows and r-travis supports os x, where would we put binary files?
• Ideally, the convention should be flexible enough that eventually I could add a binary argument to install_github(), install_bitbucket() etc. if other distributed build systems become available.

cc @romainfrancois