Move to YAML configuration for developer model

[pksohn]

Just like with #12, we should move to YAML configurations, like other urbansim models use. Specifically (these are copied from #12):

• Moving configuration parameters to explicit object attributes like in LCM objects
• Writing a from_yaml class method to instantiate Pro Forma objects from YAML, like this. Make sure the option to read directly from string (instead of file or buffer) is included.
• Writing a to_yaml class method, which depends on a to_dict method, like this.
• Writing a lookup_from_config method to wrap a lookup method for YAML, like this