PR comment for branch Rperform results

[tdhock]

https://github.com/ropensci/plotly/pull/241#issuecomment-126865718 example of automatic message with a link to test results

[tdhock]

the code they used for making a comment is at the end of https://github.com/ropensci/plotly/blob/master/inst/build-push-comment.R

[tdhock]

use branch-1-name.html, branch-2-name.html instead of always index.html

[analyticalmonk]

Opened an issue on the travis-ci tracker: Environment variable for branch from whch PR originates