explicitly specify user agent when querying user name and ref for pull request in install_github
add NEWS
Merge pull request #423 from krlmlr/405-github-pull-forbidden

Explicitly specify user agent when querying user name and ref for pull request in install_github
Merge remote-tracking branch 'hadley/master' into 388-install-gh-pull

Conflicts:
	NEWS.md

Reconciled.