Fix regular expression used to extract IDs

I ran into an issue when I tried to fetch content using a CSS selector and the `enumerateElementsWithCSS:block:` method. 

On this page : http://en.wikipedia.org/wiki/List_of_spells_in_Harry_Potter
The selector : `div#mw-content-text h3` was generating `//div[@id = 'mw']/h3`

This PR simply adds dash ("-") as a valid component, but as the valid syntax for a valid XML/HTML attribute is much more complex (according to [this page](http://www.w3.org/TR/REC-xml/#sec-common-syn)), this may need more work at some point.

It also makes `ONOXPathFromCSS` public so it can be tested, but I'm not really sure about that part.