incomplete rebase of 350-install-github to current master

original: 9f8284068812f89
move code around
add test
install latest release using user/repo@*release instead of user/repo@*
final amendments
cosmetic: remove executable bit
cosmetic: spurious Rd file change
Merge pull request #563 from krlmlr/cosmetic

Cosmetic changes
correct implementation of github_resolve_ref.github_release
test github_resolve_ref using a mock for github_GET
Merge remote-tracking branch 'hadley/master' into 350-github-releases
merge documentation for github_pull and github_release
Merge pull request #475 from krlmlr/350-github-releases

Allow installing the latest GitHub release. Closes #350.
Implement use_package. Closes #559
Add shim for library.dynam.unload
Use family, not rdname
Move find_rtools to onLoad
fix install_github command

username is missing
Merge pull request #564 from randy3k/master

fix install_github command
username is missing for roxygen in install_github doc

similar to #564, username is missing