Accept a SimpleSchema as a valid argument for the check function

In #7, @aldeed and I suggested to extend the Meteor build-in `check()` function to support `SimpleSchema` without having to call the `.match()` method on it.
While working on this PR for the Meteor core project, I've found that it was actually not needed.

This PR basically add this single line:
```js
SimpleSchema.prototype = new Match.Where()
``` 
With this, a SimpleSchema object is now recognized as a kind of "subclass" of `Meteor.Where`, and as a result it's now a valid argument for both `check()` and `Match.test()`.

Related links:
 * https://github.com/meteor/meteor/blob/devel/packages/check/match.js#L175
 * http://stackoverflow.com/questions/2449254/what-is-the-instanceof-operator-in-javascript