Add Rails 4 and Ruby 2.

Tweaks to wording in section on forward and reverse geocoding for coordinates that do not have known location addresses associated with them.

No longer do we install libs as plugins.

Make Geocoder.configure accept a hash (now preferred) and add
Geocoder.config method (for reading config). Deprecate old syntax.

Document uninitialized constant error with Mongoid.

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