Added HTTP headers to help link queries to clicks

The API now returns an X-Correlation-Id header from search requests,
which SDK clients are presumed to return with subsequent resolution
requests. This allows better performance tracking and tuning when
serving relevant content.

Search Requests can now pass placement tags which will allow for
labeling where in the app a query is occurring, and to study the
performance by tag or groups of tags.

If the AdSupport framework is included in the app which includes
URXSearch, the IDFA will also be passed in the HTTP requests.
Generalized Tags as category/value pairs

Tags are no longer directly tied to placement, but can be used in a more
general way to identify queries for the purposes of server side override
and providing dimensions to slice analytics data.
Added Result Position to resolution requests

The position of the result in the list of results given in a search
response is now passed back in the resolution requests. This lets us
know which results were resolved.
fixed up some printlns left over
removed nil check on adding X-Result-Position header
Added identifier category to X-Device-Identifier values
Added interface for resolution of arbitrary links

Now the SDK allows resolution of arbitrary URLs, not just search
results. To do this, create a new URXResolutionRequest and resolve using
the same methods you would on URXSearchResult (which is now a subclass
of URXResolutionRequest).
Standardized Copyright and added Apache V2 License

Made the copyright comment at the top of code files standard and added
the Apache V2 License to the root directory.
  make readme a little clearer
Merge branch 'master' of github.com:URXtech/urx-sdk-ios