Use Faraday instead of Patron

Hi,

I did quite big rewrite of client.rb to use Faraday library.
This give us all benefits of Faraday gem https://github.com/lostisland/faraday
The client.rb is much cleaner retry and json parsing are as the middleware.
I change default json pare to yail-ruby because is much quicker and has seamless integration flow.

We can use for example typhoeus adapater:

```ruby
require 'parse-ruby-client'
require 'typhoeus'
require 'typhoeus/adapters/faraday'

Farday.default_adapter = :typhoeus
Parse.init {:application_id => "<your_app_id>", :api_key => "<your_api_key>"} do |farday|
  # here you can pass some additionals to faraday directly
end
```