Fix duplicate serial console messages

[FoamyGuy]

Resolves #234

I am not positive, but I think the existing removeEventListener() is not working because this.onSerialReceive.bind(this) is producing a new function each time it's called. So when it gets called for remove() technically it is a different instance of the function than the one that ends up being passed to addEventListener() and thus the one that will need to be passed next time to remove in order for it to get de-activated.

This resolves it by creating the callback once, storing it on this and then referencing it from there for both the add and remove listener calls.

I tested this locally and it does seem to resolve the duplicate serial issue, with it I can now disconnect / re-connect multiple times and still end up with just a single instance of the serial output.

[makermelissa]

Thanks for the change. Let's make it private, so let's rename it to this._messageCallback. Also, please add to the constructor.

[makermelissa]

Updated my comment as using "Callback" was consistent with other usages.

[FoamyGuy]

Thank you, It's been renamed, and added to the constructor with latest commits.

[makermelissa]

Thanks