Added possibility to supply custom cluster drawables

[futtetennista]

No description provided.

[damianflannery]

Thanks, it looks good! Sticking with the builder pattern, I went the whole hog and augmented this to enable you to supply a custom ClusterSpot and configure the text displayed on the cluster (e.g. in case you wanted to show 'H' for high instead of the number of markers contained), text colour, text size etc.

I also fixed the code to properly scale the marker to the custom drawable (e.g. before if you replaced the cluster spot background with one of the purple map markers it will stretch it to fit the bounds defined in resources e.g. 32 x 32).

What's the best way to iterate... should I commit to this branch or should I fork this and provide you a patch (Inception :) )?

I still need to add javadocs but it would be cool if you could take a look first.

[damianflannery]

In the end I just powered through, merged and then committed my changes to the clustering branch. Thanks for the contribution.

[futtetennista]

Well...thank you! Haven't done that much really. I'm looking forward to using the library!