feat(promiseTracker): add register() and deregister() methods

BREAKING CHANGE:

A promiseTracker instance can no longer be created with the
`promiseTracker` function, it is a getter only now.

To migrate your code, change the following:

```js
var newTracker = promiseTracker('newTracker', options);
//..later
var tracker = promiseTracker('newTracker');
```

To:

```js
var newTracker = promiseTracker.register('newTracker', options);
//..later
var tracker = promiseTracker('newTracker');
```

Check the [project
README](http://github.com/ajoslin/angular-promise-tracker/tree/master/README.md)
for more details about this change.
feat(promiseTracker): remove events system

BREAKING CHANGE:

The events system has been removed.  That means you can no longer do
`.on()` and `.off()`.  In essence, events were a poor way of doing
'hooks' on promises.  See [this
issue](https://github.com/ajoslin/angular-promise-tracker/issues/37) for
more information.

[Open an
issue of your own](https://github.com/ajoslin/angular-promise-tracker/issues) if you
were using events and need help migrating to use anotehr method.

Check [the
README](https://github.com/ajoslin/angular-promise-tracker/tree/master/README.md)
for details on the current API for promiseTracker.
release(): v2.0.0-beta1