Make Preamble module into class; add "save" method

- Preamble module is now a class
- Preamble objects can be saved
- Preamble objects have "metadata" and "content" accessors
- Ruby 1.9 backwards compatibility
- Specs for all of the above
Update README for 'save' usage and new methods
Add more usage detail
Merge pull request #3 from wordtreefoundation/master

Make Preamble module into class; add "save" method