Document uninitialized constant error with Mongoid.

This reverts commit 65cfdfc8.

In most of these cases using anything but HTTP won't work, but this
seems more correct, and avoids having to make a change after a
provider adds HTTPS support.

This requires changing the way stubs work, so that they only simulate
actual HTTP calls, not cache hits.

Also move API key config to separate method.

Allow lookups to define a cache key (defaults to the query URL) and
define Yahoo's as the non-OAuth-encoded request URL.

Add test and display Yahoo's error message in warning.

This addresses:
https://github.com/alexreisner/geocoder/issues/322

Add bounding box optimisation

Added help text to explain that an index is required to take advantage of the bounding box optimisation

This addresses Github issue #318.

This allows selecting only the id column for cases where a single column
result is required (ie, a subquery used as an operand to the 'IN'
operator). This addresses issue #314.

Implement actual Mapquest API, not just as a proxy for Nominatim.
Add support for the premium (paid) service.

Support for Mongoid ~> 2.0.0

Doesnt crash if ::Mongoid::VERSION isn't defined.
It does fall back to Mongoid version < 3

previously the mapquest service was relying on an outdated url
that was just defering to the Nominatum service. This commit
brings in supported Mapquest API support for v1 of their API.

It supports the open (free) tier of the API along with the
licensed version of the service.

The version_x_results methods will return nil in case of an error. We
need to detect that and not execute that branch of the 'if' statement.