Luigi parameters with defaults from config.

Allow default for a parameter to be specified in a config file.
The config is lazily loaded, so it's not evaluated on parsing
(useful in the case of calling LuigiConfigParser.add_config_path
to add additional configs at runtime).

Also,
* move config into its own module
* a test utility to override configs for testing.
Merge pull request #83 from jcrobak/config-defaults

Luigi parameters with defaults from config.