Modified implementation of JSON conversions

All,

Following our last call, I implemented the new conversion scheme:

JSON to XML:

    https://github.com/orbeon/orbeon-forms/blob/master/src/main/scala/org/orbeon/oxf/json/JsonToXmlAlgorithm.scala

XML to JSON:

    https://github.com/orbeon/orbeon-forms/blob/master/src/main/scala/org/orbeon/oxf/json/XmlToJsonAlgorithm.scala

Tests (including all the examples in the spec):

    https://github.com/orbeon/orbeon-forms/blob/master/src/test/scala/org/orbeon/oxf/json/ConverterTest.scala

A related good news is that the resulting code is much simpler and
regular, almost trivial in fact.

-Erik