First pass through all fxns

[isteves]

Added basic documentation/code to all functions. Closes issue #2

TODO:

• write tests (#14)
• check that functions work (especially those that POST)
• finish vignettes (started in another branch)
• change from options to Sys.setenv

@amoeba I think I'll go ahead and merge this in (unless Travis complains) and ask for your review once I've neatened things up a bit more. For now, I've left most of the httr results without much/any modification. If we decide to, we can strip the header/etc systematically (#12)

[amoeba]

Great, these look like an awesome start. Thanks for working so hard on this!