Fix: prevent downloading of tiles multiple times.
Added tests for tiled image demo view.
Allow customizing zoom step.
Standardized file headers and documented code.
Synchronize operations.
Use CGFloat/NSInteger insetead of float/integer.
Cleaned up interactive demo a bit.
Zoom will pan towards the point being tapped.
Merge pull request #20 from dblock/tiled-map-view

Tests for tiled image map view controller + bugs + docs + zoom.