Helper method to easily select a single element via CSS

[Dan Watkins]

I often find myself writing code like this:

elements = parent_element.select('...')
self.assertEqual(1, len(elements))
element = elements[0]

Being able to do:

element = parent_element.select_one('...')

would make this much more expressive.

[Leonard Richardson (leonardr)]

Added in revision 371. This has brought to light a naming inconsistency: find()/find_all() find one/many tags, but select()/select_one() find many/one tags. But it's way too late to change those names.