Add type option to artist hotttnesss method

Allows a type to be specified for an artist's hotttnesss metric:

```ruby
artist = Echonest::Artist.new(API_KEY, 'Radiohead')

artist.hotttness
artist.hotttness(type: :overall) #default
artist.hotttness(type: :social)
artist.hotttness(type: :reviews)
artist.hotttness(type: :mainstream)
```