json was introduced *in* 2.6
with Python 2.5 support dropped, simplejson is not longer requred
rename DictMixing -> UtilityMixins
Make Software a top level entity

Removes the AgentSoftware and JobSoftware tables and replaces them with
a single "Software" table. This table is referenced in many to many
relations by Job and Agent.
Use DictMixins in Software model

To get to_dict() and to_schema methods
Merge branch 'software_as_top_level_entity' of github.com:guidow/pyfarm-models into software_as_top_level_entity

Conflicts:
	pyfarm/models/agent.py