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