Add simple XML support.

[wtrocki]

API Mapper currently only supports JSON-based APIs. The main feature request we have received from community is to support general XML-based APIs too. This requirement is just for pointing directly at an XML source without any schema like XSD.

For simplicity we can map existing xml to json and work with json format. This approach would be acceptable for most platform usages and it would allow us to interact with javascript objects instead of plain XML.