Fix: Allow multi name resources

Previously, when a resource was defined as follows:

    resource :admin_users

A load was attempted on the `Admin_users` class.  It would be more
valid in ruby to look for a class called `AdminUsers`

This is now the case, active_support/core_ext/string has been included
to facilitate this change.
Feature: Use singular class name on a resource

When defining a resource, the singular class name is preferred to be
more in line with rails.

    class User < Rapidash::Base
    end

    class Client > Rapidash::Client
      resource :users
    end

This will set up a valid link between the client and the User class.
If the singular does not exist then the plural term can be used.  This
will raise a deprecation warning but will not break.

It will be a requirement in the future to let a user override the class
when defining a resource:

    class Client > Rapidash::Client
      resource :admin_users, :class_name => Users
    end
Fix: Use Object.const_defined instead of Kernel

This fixes checking const_defined? for ruby 1.8.7

Resources:

 * http://www.ruby-forum.com/topic/144313
Cleanup: Update documentation to use Singular class names

[ci skip]
Merge pull request #7 from Gazler/multi_name_resource

Fix: Allow multi name resources
Moved clientable module to client class

Added simplecov

Extracted extension to Client class, added normalize method which append extension to url

Extract site methods to client

Raise error if site nil

Marked client request methods as protected

Write readme about tests and coverage
Merge remote branch 'mlimax/extract_general_functionality_to_client' into development
Fix: Ensure simplecov does coverage of all the files

and not just the spec files.
Cleanup: Remove clientable file as the module no longer exists

For all other client based modules, isolate the tests so that they only
test code in their own modules and do not rely on extending Rapidash::Client

Some additional tests have been added to test the new methods in the Client
class.