Add URL and HTMLURL to Issue

Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>
Implement support for RepositoriesService ListContributors
add Chris Schaefer as contributor
travis: test on go 1.2 and tip
Add support for RepositoriesService ListBranches
Make HTMLURL accessible on user struct
add Tobias Schmidt (of SoundCloud) as contributor