fix attempting to draw with recycled bitmaps

Fixes #53
Add a clearMemory method to ImageManager and BitmapPool
add test to ensure recycle not called on bitmaps evicted from cache
add basic tests for bitmap pool
add trimMemory method to bitmap pool
add trimMemory method to image manager
update gradle build file
handle exceptions loading from disk cache
add a toString method for flickr photo objects
Merge pull request #6 from bumptech/master

auto merge