Added Xml Parsing support in Web component

This branch enables parsing XML content from many API requests. For details, please refer to the proposal.

Proposal: https://docs.google.com/document/d/1_R_lKg2dzrBUeCn6luLOSVL2iwWlXFlMKejOpBm-g68/edit?usp=sharing

Try it on http://xml-demo.appspot.com with its [Companion app](https://www.google.com/url?q=https%3A%2F%2Fdrive.google.com%2Ffile%2Fd%2F0B6pgfXvNtaFyT2I1WmJqYUxRTGs%2Fedit%3Fusp%3Dsharing)

Demo project: https://www.google.com/url?q=https%3A%2F%2Fdrive.google.com%2Ffile%2Fd%2F0B6pgfXvNtaFyTFRtUk4wZmNYcUU%2Fedit%3Fusp%3Dsharing

Post: https://groups.google.com/forum/#!topic/app-inventor-open-source-dev/kBdil_Z6OEA