Implementation of config parser validation and completion

This PR implements a new file that describes the config file with all options. Once such a file is read in it can be used to validate an actual config file. It completes default values and gives back a `dict` with all default values included and validated. 

WIP

 - [ ] py.test readable tests
 - [ ] an actual default config
 - [ ] a documentation page. 

**OPTIONAL**
 - [ ] conversion of a config validator to a documentation page