Use native DOMParser and XMLSerializer

[nodaguti]

After #25, I've made a further progress in the direction of removing xml-js, migrating from a xml library that provides their original interface, to Web standard DOMParser and XMLSerializer.

With this change the package size with required dependencies will be reduced by ~128KB.

While DOMParser and XMLSerializer are supported since Chrome 4, for environments that don't support DOMParser and/or XMLSerializer, there are polyfills like xmldom and jsdom. xmldom is a lightweight (30KB) pure JS implementation of DOMParser and XMLSerializer with no dependencies on Node.js standard modules, so I use it in the updated tests to verify that our vast/vmap parsers will work well with xmldom.