Update documentation to show more examples.

[vjustov]

Readme should ilustrate more examples:

• Usage and worst, best scenarios using regexes.
• How to use optional tokens

And some other things that are on the google group but are not documented on de readme.

[mxygem]

@cjheath What happened to the documentation that was on rubyforge? I'm only just now discovering Treetop for a project I've just started and could use access to more data. :) Do the docs exist anywhere else?

[mxygem]

Nevermind! Found the directory for it, derp!