fixes #150: generate Contracts class

[robnormal]

To generate the class statically, use bin/generate-contracts. This generates the "standard" Contracts class. To override code, you will have to use the admittedly awkward setup below. Contracts.configure(:contracts_use_file => false) causes the class to be generated on the fly and eval-ed, instead of loading from the existing file.

As an example, including the following at the top of spec/spec_helper.rb fixes #14:

module Contracts
  @load_on_require = false
  require 'contracts'

  Contracts.configure(
    :contracts_generator => Contracts::ContractsGenerator.new(
      'Class' => "require 'rspec'; lambda { |arg| arg.is_a?(contract) || RSpec::Mocks::Double }"
    ),
    :contracts_use_file => false
  )
  Contracts.load
end

The template for contract.rb lives in templates/contract.text.erb. ContractsGenerator does the generating. So far, I've broken out only the part I needed to override - namely, the lambdas used to check Class and other basic contracts.

[alex-fedorov]

@robnormal can you get rid of require_relative ? (travis failure) (load should be fine in this case)

@egonSchiele can you take a look at this one? I am still on the fence and have no real opinion on the API that we want to provide for such code inlining and overriding.

[egonSchiele]

@robnormal thanks for taking the time to do this PR. I don't like the pre-compile step because the code is getting pretty complicated (I appreciate that you made a minimal amount of changes, but it's still very different from most ruby projects). I'm afraid this is going to make it hard for newcomers to understand the code. So it will slow down contribution and monkeypatching. I think I would rather find a traditional solution, even if it means adding a few more methods and marginally slowing things down.

[alex-fedorov]

@robnormal I have an idea, how your problem can be fixed without code-generating, I will create a PR and will use your case as example soon.