Pivotal tracker reporter

Hi,

I've implemented a Pivotal Tracker reporter.

I've found and fixed a bug in [httpBodyDataForDictionary](https://github.com/jeanregisser/Shakedown/commit/b1f331a24c4f13514fc1046dfbeaa75994ed7f19).

I added a simple generic XML parser that could be reused for other reporters.
The code of the XML parser was adapted from https://github.com/genkernel/XML-to-NSDictionary. I've added proper attributions to the SHDXMLReader.[h|m] files.

Also we might want to move [SHDPercentEscapedQueryStringFromStringWithEncoding](https://github.com/jeanregisser/Shakedown/pull/new/pivotal_tracker_reporter#L1R15) to the base reporter class so that it can be reused.

Let me know what you think.