Support for X_REAL_IP and X_FORWARDED_FOR

For discussion see:
https://github.com/alexreisner/geocoder/issues/329

Fixes the problem described here:
https://github.com/mongoid/mongoid/issues/2579

This should be reverted when the Mongoid issue (or whatever it is)
is fixed.

Geocoder should be required after other things like Rails and Mongoid.

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