Tokenize 'this' as a keyword

Per the specification, `this` is a JavaScript keyword that is forbidden
from being used as a variable or property name. So we should tokenize it
as a keyword and not a variable.

See: http://coffeescript.org/documentation/docs/lexer.html#section-69

Fixes #36
Remove unnecessary parens
Change this to be tokenized as a special variable
Merge pull request #39 from lee-dohm/this-keyword

Tokenize 'this' as a keyword