Use Json for configuration files

This patch converts the current configuration files to json and uses
the python json library to parse the them. Json is a standard format
and as such already has many encoders and decoders available. This
option also leaves room for a schema validation using something like
jsonschema.
Use duck typing over strict type verifcation

The json library uses unicode strings over standard ascii strings and
thus strict type checking against strings will not work. This patch
removes those checks as well as other type checking as Python
generally uses the duck typing strategy over strict type verification.

In these cases, a type error represents a schema error and should fail
loudly. In the future, schema checks for the configuration files will
provide a better system to validate the configuration files.