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%2F0B6pgfXvNtaFyYnAwSUF5TVZyMFU%2Fedit%3Fusp%3Dsharing)

Demo app: https://www.google.com/url?q=https%3A%2F%2Fdrive.google.com%2Ffile%2Fd%2F0B6pgfXvNtaFyUEZHRU03ekxOVk0%2Fedit%3Fusp%3Dsharing