Data.gov API for available datasets

[allenhai]

There should be a standardized API for listing the various datasets available. People should be able to download a .csv, for example, programmatically through a GET request.

[rebeccawilliams]

Hi @the-len, care to weigh in on these relevant open issues:

#180
#294
& to a lesser extent #315, #398, #420

[philipashlock]

@the-len There is some basic information about our API endpoint at https://www.data.gov/developers/apis

Here's an example Data.gov API request:
http://catalog.data.gov/api/3/action/package_search?q=rain

Or to get all datasets:
http://catalog.data.gov/api/3/action/package_search?q=:

If you have suggestions for improving the API documentation, please add them to #180