TypeScript rewrite and jsdom support

[backslash47]

Hello, this is not an issue per se.

I just want to let you know I've

• rewritten the codebase to TypeScript
• added support for jsdom (xmldom is still supported)
• rewritten the tests to Mocha+Chai
• changed XPathResult.stringValue and other similar methods to getters for conformance with https://www.w3.org/TR/DOM-Level-3-XPath/xpath.html#XPathResult

You can find my repo at https://github.com/backslash47/xpath . I published it to npm under xpath-ts for now. If you are interested, please try it as a replacement (beware of the stringValue instead of stringValue() )