add [] syntactic sugar to Assignment object

previously one had to do params.get(.) and params.set(., .) to get and
set random variables. Now you can simply say params[.] and params[.] = .
bug fixes to Experiment::get() in ruby
update to ruby Experiment class and demo

-initialization more closely mirrors that of the Python version.
-Created an example of a prototypical experiment that produces
assignments consistent with the Python version.

README.md has been updated to reflect this.
added simple example
Merge pull request #21 from facebook/ruby-with-decorator

more functional ruby port