[minimongo] Add support for bulk insertion of an array of models

I noticed that the minimongo package does not support the bulk insertion of models when `insert` is passed an array of documents. This is a standard mongodb feature as of `2.2`.

I am not sure if this is the way you will want this done in the long term, and someone (or me) still has to make `Meteor.Collection` conform. Nevertheless, this is a simple starting point which introduces parity between minimongo's `insert` and the standard mongodb and server-side driver's implementation of that function.

More information will be available in an issue I will file immediately after submitting this pull request.

Cheers