Initial commit/skeleton for Flavor model

Initial code for the Flavor model. Also starts to use wsattr for validation (thanks @mtaylor).

You can create a flavor with:

    curl -X POST -H "Accept: application/json" -H "content-type: 
    application/json"  -d  '{"name" : "large", "capacities" : [ { "name"
    : "cpu", "value" : "1", "unit" : "count" }]}'  http://localhost:6385/v1/flavors

TODO:
1. get the capacities to serialize/display when doing a GET.
2. set the ID for flavor as UUID String (to be done for all models eventually) - getting errors now.

marios