Edits to install2.r and preliminary installGithub.r

Hi Dirk,

Here's the PR we discussed.  I did these in separate commits in order of increasing probability that you'll reject them :-)   

- First commit just lets errors be errors by commenting out the offending `q(silent=TRUE)`

- Next commit adds dependencies line to install2.r.  I wasn't sure if the full command-line argument name should match the R argument (`dependencies`), but I just went for the more convenient `--deps`. I also boldly changed the default behavior from TRUE to FALSE, since I think TRUE can cause trouble (e.g. cyclic dependencies, or just trying to install a bunch of suggested packages that might need certain systemRequirements satisfied first, and thus causing an install to fail when it need not have).  However, clearly changing this default could break someone's code, so maybe I shouldn't have done that.  

- I took a stab at writing an installGithub.r (not sure if that's an appropriate name, seemed like camelCase was convention for the examples though).  I wrap `devtools::install_github`, since I'm sure hadley has done a better job than I would in writing & maintaining such a function, not sure if that's what you had in mind though.  

Anyway, lemme know what you think.  Am enjoying docopts immensely.  

Cheers,

Carl