Improved zoom animation, configurable tile dimensions, pan and zoom listener, off-screen loading support

• #127 If max tile dimensions have been set and dimensions are known, load base layer without waiting for onDraw. This improves ViewPager support - the next image can be eagerly loaded.
• #148 Revised behaviour of zoom gestures as the image fills the screen for a smoother animation.
• #171 Added a new image state listener class to allow activites to receive pan and zoom change events.
• #206 Allow maximum tile dimensions to be set, so performance and memory usage can be optimised when the device supports excessively large tiles.
• #211 Made double tap tolerance density aware for improved high density screen support.
• #216 Ensure cursors are closed after exceptions.
• #222 Added image event for preview released so it can be recycled.
• #245 Avoid NPE if view has been detached from parent.
• #253 Corrected position of tile background colour for preview image.
• #262 Method to check whether an image has been set.