Beginning work on Rails 4 version

Modified rakefile to accept Rails 4.0
Downloaded and added tinymce 4.0.2. Using it for Rails 4.0 and old versions for Rails 3.x
Updating README.markdown
Changed README.markdown to be a target for README.markdown.in so I can dynamically include lib/use_tinymce/version.rb
added lib/usr_tinymce/version.rb to centralize versioning
A useless checkpoint comment

Added assets/use_tinymce_init_jquery_4_0_2.js to hold differences with 3.x version
Updates to README.markdown.in, README.markdown, the rake file.
Added Rails.versison == 4.0.x to railengeine.rb so it actually gets installed
Rewrote 'use_tinymce_link' to handle changes in paths to javascript introduced
in TinyMCE 4.0 - while looking to the future for more incompatable changes