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()`.