Feature: ability to remove session event listeners in session scripts.

[buffermet]

In session scripts we are missing the option to remove session event listeners. If we have a function like removeEventListener(tickerName, func) then we won't get a buildup of unused session event listeners.

Example:

var myFunc = function() {
    // do stuff
}

onEvent('ticker.myTicker', myFunc)

// later we could decide to remove this listener like so:
removeEventListener('ticker.myTicker', myFunc)