Add example to README on retrieving coordinates from mongodb model.

Fixes issue #75 (ActiveRecord) and more (Mongoid).

Prevent an exception when geocoding is initialized for
a child STI model when the parent is also geocoded.

In certain conditions a lookup returned [nil] instead of [], which
caused a Result object to be initialized with no data.

Allows custom handling of geocoding service connection problems.

Fixes #56

Was causing problems when installing with bundler from Git URL.

Instead use a proxy if http_proxy or https_proxy is set. Also revise
code style to be more consistent with rest of library.

Geocoder will try to use environment's http_proxy or https_proxy when doing lookups unless that behaviour has been disabled in the configuration

Specify path to VERSION file relative to lib/geocoder.rb.

Child classes didn't have access to parent's configuration. This is kind
of crude (if geocoder_options not defined, see parent) but it works.