Added support for URLs with placeholder arguments

In Sinatra, you can specify a URL pattern handler such as /object/:id that will match URLs like /object/1 or /object/5.

I added support for this type of URL to be specified in the URL argument to url_for.  If url_for's options argument has a key that equals the placeholder, the value associated with that key will be substituted into the url and that argument is removed from the options.

I added 2 rspec examples that show how this works.
There's a bug here - method url_for sets options to nil unless options was passed in. So if you don't pass in the optional parameter options you get "undefined method each_pair for nil:NilClass". BTW - the bug above is present in the version obtained from rubygems.org via 'gem install sinatra-url-for"