configuration helper file

This addresses #2 

Instead of making it a Railtie I just made it a single ruby file that is not automatically included.  So in rails projects you just add a single line the spec_helper.rb file like this:

```ruby
require 'rspec/rails_rabl'
```

That sets up the configuration that automatically includes the helpers for specs in <code>spec/views/*</code> and extends the helpers for <code>rabl_data</code> <code>rabl_template</code> <code>view_path</code> and <code>rabl_config</code> that goes inside your <code>describe</code> blocks but outside your <code>it</code> blocks.