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.
Something that might make this patch simpler would be to have the tryGetPreferredMatchMaker function return just a string with the preferred MM and return undefined in case of being unsuccessful. In other words, instead of returning an object in the format: { found: bool, matchMaker: "myMM" } it would return: "myMM" in case a matchmaker was found, and undefined if no MM is found... then this if statement could be:
```
if (result !== undefined) {
   return result;
} else {
   return "flat"
}
```

Or even:

```
return (result) ? result : "flat"
```