Provides XPathContext to extend the query capabilities of XPath

[zmjack]

Fixes #144
I've been looking for ways to extend xpath query capabilities, and I think it's very necessary.
I raised this question at the beginning of last year. But I'm sorry, this solution comes too late.
Some classes was first released in Dawnx library, I improved it to make it easier to use, and it is now available to HAP under the MIT License.

[JonathanMagnan]

Hello @zmjack ,

Your current code cause some problem with licensing.

The code is taken from Dawnx with is Apache 2.0: https://github.com/zmjack/Dawnx/blob/master/LICENSE.md

[zmjack]

OK, I re-uploaded and renewed its license.

Before that, the version on github was not the latest.

Now, it's using the MIT license.