Introduce Socrata search functionality

[tomschenkjr]

Socrata's new Discovery API allows for domain-specific and network-wide searching. That is, to search for data for a particular portal (e.g., data.cityofchicago.org) or to search across all portals that use Socrata (e.g., NYC, SF, etc.).

The results of the query should be:

1. Easy to manipulate and navigate within base R data structures
2. Can construct queries using familiar argument structures and the function will compose a valid query
3. Handle Socrata query protocols and limitations -- such as throttling and paging
4. Interoperate well with other RSocrata functions, for instance, using the search function to find data sets and then calling read.socrata to download each data set found in the search.

The result of this might appear like a new search.socrata() function, akin to:

search.socrata(domains = ..., # vector/string
  categories = ..., # string
  tags = ..., # string
  asset_type = ..., # vector/string, referred to as "only" in discovery API docs
  attribution = ..., # string
  text = ..., # string, referred to as "q" in discovery API docs
  derived_from = ..., # string
)

More details on each of the fields can be found here