If you put an alert in a callback `this.pressing` is always wrong

[andyhmltn]

If you use Quay in the following fashion:

myQuayInstance.capture({
    'up': function() {
        alert('Up up up');
    }
})

an alert pops up. However, unless you hold on to up until the alert is dismissed, the keyup event is never registered and up is forever in this.pressing

I'm unsure if this is an issue or not as it only affects people using alert in callbacks. This is a result of me removing this.pressing = [] after a callback is run.

1. Allow for holding key(s) and pressing another key. For example: Pressing cmd+alt and mashing a would run the cmd_alt_a callback on every press of a

OR

2. Use alerts in your callbacks.

Pick one. It seems to me that there is probably a better way of allowing for 1 that doesn't destroy 2.