closes #213: merge the documentation of runUrl(), runGithub(), and runGist()

removed the port and launch.browser arguments, and used ... instead
move runGist() and runGithub() after runUrl(), and merge their descriptions into the description of runUrl()
let download() fail in this case (ref=NULL); since ref='master' by default, if the user provides another value, he/she is expected to have read the documentation
the name variable is not used anywhere
closes #427: runGitHub() can accept "username/repo" in its first argument
the message is not very useful, given that download() will emit a message
subdir=NULL is the default of runUrl()
Merge pull request #507 from yihui/doc/runUrl

Merge doc for runUrl(), runGist(), and runGitHub()