Routing::GeoPoint#fetch

The method is implemented so it is compatible to Hash#fetch. This way a Hash can be used in place of a Routing::GeoPoint and vice versa.
Routing::Adapter::RestAdapter utilizes Routing::GeoPoint#fetch now
Changes the implicit dependencies an Adapter has

Before, an adapter assumed that an object responds to #lat and #lng.
Now, an adapter assumes that an object responds to #[](key), where key
is :lat or :lng.
Version bump: v0.1.0 -> v0.2.0
Release v0.2.0