fix(utils): add the ability to prevent the default action of onEvents

[poxrud]

Add the ability to prevent the default action of onEvent (onclick, onpaste, etc..) from occurring
by returning false.

For example, you should be able to stop text from being pasted into a textbox with:

textarea.onpaste = function() { 
  return false;
}

I discovered this during my investigation of issue #232, which this PR fixes.
I suspect it will also fix issue meteorhacks/zones#80

[poxrud]

@mhevery
This PR is now incompatible with POST-v0.6.0., however I've verified that this issue still exists.
Should I submit a new PR for this?

[mhevery]

Yes please!

[mhevery]

Any update on a rebase?

[mhevery]

Actually, it applies cleanly, and tests, pass so I am merging it in.