add support for DOM selectors

[lknop]

This PR adds possibility of referencing fragments by id, class or tag name, as outlined here: http://www.thymeleaf.org/doc/articles/layouts.html#including-with-dom-selectors

Includes also fix for #5 .

[markhobson]

Thanks for the PR @lknop. I've pushed back a slightly different approach:

• I've only supported the explicit DOM selector syntax templatename::[domselector] for the moment. This is so unmatched fragment names don't inadvertently match other elements via DOM selectors. I'm open to adding support for the implicit templatename::domselector syntax as a fallback if the fragment name doesn't match though.
• I fixed #5 with regexs and added tests.

Apologies for the delay again in getting around to this!

[markhobson]

I've raised #8 to cover the implicit DOM selector syntax.