Fix for last modified column not existing on a new model and custom headers

These two pull requests have a fix and a feature.
1) fix the SQL error caused by last modified column not existing on a new model
2) Allow user to specify a set of custom headers. This allows things like StackMob API keys to be passed. E.g.
```
        "headers": {
        	"Accept": "application/vnd.stackmob+json; version=0",
		"X-StackMob-API-Key": "your-stackmob-key"
		},
```