Attempt to fix dev_help bug when not in rstudio
Document with latest version of roxygen
Add user agent for httr requests
Merge pull request #384 from krlmlr/c2d4u

install dependencies from c2d4u
Really fix rstudio_has
helper function github_parse_path
repo argument to install_github now also supports passing subdir, ref and pull
use valid tag devtools-1.4 instead of invalid branch v1.4-rc
stop if GitHub path is invalid