Reformat autocomplete's API docs.

Add an `enableCache` attribute to AutoComplete. Set this to false if you want to disable caching in built-in result sources.

Make requestTemplate available to YQL sources as {request}. [Fixes #2531285]

`this` should be the AutoComplete instance in requestTemplate functions. Includes a bonus fix for an issue that caused the requestTemplate function to be called twice instead of just once for XHR requests.

Use expanded request string as cache key instead of just the query. [Fixes #2530410] This ensures that the cache key is based on the entire request (including any extra data provided by a requestTemplate) rather than just the query (which may be shared between requests with differing requestTemplate params).

<select> nodes may now be used as result sources.

Use a simple Y.mix() for autocomplete-sources instead of Y.Base.mix(). Y.Base.mix() was working, but was actually being used incorrectly in this case, since the receiver (AutoCompleteBase) doesn't actually extend Y.Base. It only worked because autocomplete-sources had an empty constructor, so there were no side effects from the fact that the constructor was never executed.

Add a sourceType attribute, which may be used to force a specific source type, overriding automatic source type detection. [Ticket #2529974] This change also makes it possible for new custom source types to be added at runtime by third-party code in the same way that they're added by autocomplete-sources, simply by mapping an sourceType name in Y.AutoCompleteBase.SOURCE_TYPES to a source setter function. I haven't yet documented this pluggable functionality since I'd like to let it simmer a bit before setting it in stone.

Pass the query and the requestTemplate output separately when sending a request. [Fixes #2529895] Also fleshed out the unit tests for sendRequest and for the various autocomplete-sources source types.

Refactor for clarity.

Really set allowCache to false this time.

Disallow YQL caching. This is a temporary workaround for a problem created by the fact that JSONP uses a single callback per instance regardless of URL, YQL reuses a JSONP instance across multiple requests, and AC reuses a YQL instance across multiple requests.

Add a yqlEnv config attribute to allow the use of custom environment files with YQL sources.

Add support for XHR-based sources without using a DataSource. Like JSONP sources, XHR sources are specified by setting the source attribute to a URL. Any URL string with a {callback} placeholder will be treated as a JSONP source; a URL string without a {callback} placeholder will be treated as an XHR source.

Move code for JSONP and YQL sources into a separate autocomplete-sources module. The autocomplete-sources module is included in the autocomplete rollup, but is no longer part of autocomplete-base, which results in a smaller base module when the extra sources aren't needed.