CDATA filtering options, new item addition functions, unit tests

* Allowing selective disabling of CDATA wrapping by field name.
* Adding items by concatenation of exiting items of by replacement
* Associated unit tests
* example code
* Readme updates.

This is similar to existing PR #28, but allows for finer control over what fields can be skipped for CDATA wrapping.
I now use a date mocker which always sets this to `Wed, 10 Dec 2014 19:04:57 GMT`.