Refactor

This patch splits Utils.__init__.py into a number of more focused and maintainable modules.

There should not be any code changes in this patch, only moving code around.

For now, all references and unit tests are left unchanged to proof the change did not break too many things.

But a clean up will follow afterwards.

Summary of this patch:
* Perfdata related things > metrics.py
* Git related things > git.py
* send_nsca > nsca.py
* All decorators > decorators.py
* PynagError > pynag/errors.py
* reconsile_threshold > Plugins/new_threshold_format and renamed to convert_to_classic_format()

Please take a look and provide feedback.