Added default option to load default config files to cli and core.

Also made the api to load yaml files more consistent, and fixed a bug
where the initial loaded configuration (with specified config={}
argument) was forgotten after a reload.

both in core.py and config.py. The api is more consistent this way.
Add MXVersion, for comparing mendix runtime versions
Added version parsing to config.py
Merge branch 'apification-versioning' into develop