install packages from c2d4u

- still install roxygen2, rstudio, rbenchmark and testit from source
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
Update NEWS
repo argument to install_github now also supports passing subdir, ref and pull

- helper function github_parse_path, stops if GitHub path is invalid
- update parameter documentation, roxygenize
Merge pull request #380 from krlmlr/376-install-github

enhance repo parameter to install_github
Make wd() work. Fixes #374
Also update docs. Oops
Fix overly specific test
Tweak test that was failing because of quote types
Bump version
Add description of rcmd notes
Bump version
Merge remote-tracking branch 'upstream/master'