Set custom timeout limit per describe block

The issue was that the binding to the suite obj inside of describe blocks was being lost because of the `Meteor.bindEnvironment` wrapping. Mocha binds the suite to the describe function block here: https://github.com/mochajs/mocha/blob/master/mocha.js#L1001.

I made a workaround so Mocha's binding could get through `Meteor.bindEnvironment`.

Not sure about how the function should be called, so I went with `wrapRunnable`.

Fixes https://github.com/mad-eye/meteor-mocha-web/issues/82.