Changes to core for the new services code
New XML generating code.

This introduces dynamic generation of XML commands so there is
no need for most of the constants at the end of core.py. Adds
a new prettify utility to utils, and fixes a bug in __init__.py
(Exceptions have moved)
Service docstrings
Merge branch 'new-services' of https://github.com/lawrenceakka/SoCo