Dispatcher

 - Added preference-based dispatching. Looks for the active context with the highest priority with a "http://registry.gpii.org/common/matchMakerType" preference and dispatches to the matchmaker specified there.
perhaps clearer with:
```
if (matchMakerPayload.preferences === undefined || matchMakerPayload.preferences.contexts === undefined || 
    matchMakerPayload.preferences.contexts.activeContexts === undefined) {
    return {
       "found": false
    }
}
```