Adding support for MaxMind's local database.
Update tests to handle maxmind_local specifics.

The result of the lookup in maxmind_local is stored at test/test_helper.rb to avoid the inclusion of maxmind's database in repository.
Adding geoip gem to gemfiles/Gemfile.mongoid-2.4.x.

Required for ruby 1.8.7 and 1.9.2 builds on travis.
Improving documentation to mention about the geoip gem requirement.
Small typo fix in README file.
Move geoip gem check to the initialize function.

Lookups are always loaded even when they aren't used. So the geoip gem was being checked even when user wouldn't like to use the maxmind_local lookup.
Rename maxmind_local test file because it was missing the _test suffix.
Update README file to show the correct way of using maxmind_local lookup.
Adding latitude and longitude to maxmind local result.
Change maxmind local to return an empty array when nothing is found.

Previously it was returning an exception which is not the same behavior as the other lookups.
Cosmetic change to README file.

Change maxmind_local's sample code to another file.
Merge branch 'maxmind-local' of git://github.com/fernandomm/geocoder into pr/462

Conflicts:
	README.md
	lib/geocoder/lookup.rb
	test/test_helper.rb