Feature/validation xml

Hey,

Here is a module that enables XML validation. I've noticed that there is no simple way to compose XML validation rules in Scala, so this module could be a great help for those who deal with XML.

I've added (through implicit classes) 2 operators to Path: "@@" for attribute picking and "@=" for child node picking while filtering on an attribute (see the tests for the use cases).

What do you think?

Cheers