Added sibling selectors ('+' and '~') support

Added:
* adjacent sibling selector (CSS2.1, '+')
* general sibling selector (CSS3, '~')

http://www.w3.org/TR/selectors/#adjacent-sibling-combinators

Like the '>' selector, both work only if second operand is a tag-name (e.g. `'p + a'` is ok, `'p + .c'` is not ok).

Thanks for this addon. I don't see how BS could once be shipped without it.