Refresh documentation

[c42f]

At this point with a bit of experience with using this library, the docs should be modernized a bit, including some of the following:

• Document the data REPL (and anything else from the JuliaCon talk I'd forgotten to get in the docs!)
• Document the new URI-like form of arguments passed to dataset()
• De-emphasize the high level @datafunc — perhaps we'll remove it in the future?