Add tile loading events to image tile sources

[tschaub]

This adds tileloadstart, tileloadend, and tileloaderror type events to tile sources.

[tschaub]

Here's the example demonstrating a simple progress bar monitoring tile loading for a single source:
http://tschaub.net/ol-tile-load-events/examples/tile-load-events.html

[bartvde]

This looks like a solid piece of much welcome work @tschaub
LGTM

the only remark I have is a typo in one of the commit messages (Uregister -> Unregister)

[tschaub]

Thanks for the review @bartvde. I reworded that commit message. Will merge when Travis goes green.

[tschaub]

Now that we register an additional listener for tile events, it will be nice to avoid creating tiles that are not needed. These listeners aren't unregistered until the tile cache for a source is full. See #3246 for a change that avoids unnecessary tile creation.

[bartvde]

hey @tschaub forgot to ask, but do you have plans to add events for image sources as well or not? If not, I can probably take a crack at it.

[tschaub]

@bartvde yes - I'd like to do the same. But I'm not working on it currently, so it would be great if you wanted to take it on. If I do find some time for it, I'll open a ticket first to indicate that I'm working on it.

[elemoine]

Nice work @tschaub.