Initial resource class work

Initial work to for resource class creation.  

N.B. We still need to add in relationships between resource class and flavors and racks.  This will follow in subsequent pull request.

N.B. Be sure to remove existing sqlite db and run: 

```
$ tuskar-dbsync --config-file etc/tuskar/tuskar.conf --debug
```

To test manually, run server:
```
$ tuskar-api --config-file etc/tuskar/tuskar.conf
```

To create resource class:

```
$ curl -H "accept: application/json" -H "content-type: application/json" -d '{"name":"foobar", "service_type":"compute"}' http://0.0.0.0:6385/v1/resource_classes
```
To list all resource classes:

```
$ curl -H "accept: application/json" http://0.0.0.0:6385/v1/resource_classes
```