Implement $.parseHTML

From the [jQuery API
documentation](http://api.jquery.com/jQuery.parseHTML/):

> $.parseHTML( data [, context ] [, keepScripts ] )
> Parses a string into an array of DOM nodes.

The `context` argument has no meaning for Cheerio, but it is maintained
for API compatability.
Re-factor $.parseHTML to use $.load
Add missing unit test description
Merge pull request #231 from jugglinmike/parsehtml

Implement $.parseHTML