Allow more content types

addresses issue #2

To avoid errors on content-type of api when valid json response changed to catch error on JSON parse. 

Sample code for api that returns text/javascript. 

````ruby
require 'rapidash'

class Stores < Rapidash::Base

  def store!(owner)
    self.url = "/#{owner}/store"
    call!
  end
end

class Client < Rapidash::Client
  method :http
  resource :stores #An array can be passed through
  extension "js"
  site "http://api.bigcartel.com/"
end


client = Client.new
p client.stores.store!("ugmonk")
````