Adds pry to development and creates a Rakefile

* Rakefile describes a `console` task to load a pry console and require
  the gem automatically, to interact with current code.
Fixes 3 of 4 specs on Conekticut::Client::Payment

* Pending: Add a method to take care of tokenizing a credit card via
  Ruby, without depending on Conekta's JS library.
Drafts Card and Tokenizer classes.
Implementing a valid Tokenizer handler.
Fixes card payment spec
Arranges dependency loading.
Removes unnecessary config options on Tokenizer specs.