Adds Focus, View, Degree filters and `descendants` association call to gem 

* Allows passing the `focus` option to `GeoPlanet::Place.search` calls

`GeoPlanet::Place.search('Toronto', focus: :ca)`

This helps, precisely with Toronto, where a simple query `Canada,Toronto` provides different results from `Toronto,Canada` by removing the term order ambiguity

* Adds `descendants` association call

`GeoPlanet::Place.new(560743).descendants(count: 0)`

This returns all the descendants (children, children of children) for a given place with one call