Map preload (WIP).

Map tiles near the event location begin to preload when EventActivity is opened. When the user leaves, the preloading stops. Before preload there's a check, what map tiles are already in
the disk cache. This takes about 1 ms for every tile, which can be slow when there are lot of tiles.
Add map preload.
Merge branch 'master' into tomucha-na-speedu

Conflicts:
	coordinator-android/src/cz/clovekvtisni/coordinator/android/event/EventActivity.java
	coordinator-android/src/cz/clovekvtisni/coordinator/android/event/map/view/NetworkTileLoader.java
	coordinator-android/src/cz/clovekvtisni/coordinator/android/util/DiskCache.java
Finished merge.