Add parameter binding

Bind a single callback to a symbol that replaces the value passed to the
on() closure.

Fixes #31
Added caching of bound values for access from other binders

The theory behind this is when you have a nested resource url like
/users/:userId/posts/:postId. You need to access the bound :userId in
your bind() for :postId. This places the bound value in a static
variable which can be accessed via bind('userId').
Merge pull request #32 from rmasters/param-binding

Add parameter binding