(feat) adds promisified constructor, fixes #10 and #11, bumps minor version

[sunny-g]

In reference to #10 and #11, this restores the observer methods added, changed and removed to the observer object and calls them when handling their corresponding DDP messages.

This also fixes a mismatch between the naming convention changed and updated that wasn't resolved in #11; the observer now has a method changed by default that is called when receiving DDP messages of the type "changed".

[codydaig]

LGTM

[sunny-g]

Closing because I see why you made the changes (minimongo lets us observe collections rather than directly observing stored data from subscriptions).

[sunny-g]

I've removed the observers since they can't be used, regardless of maintainCollections, and added another class DDP that is the same but with Promise-ified versions of all of the asynchronous functions.

There may have been a thing or two I've forgotten so let me know if there are any changes you want me to make.

[yogiben]

+1