Restore keeping track of desired drawable alpha

- https://github.com/square/picasso/issues/25
- 18c8427 broke alpha support on PicassoDrawable
Merge pull request #557 from UweTrottmann/fix-alpha-support

Restore keeping track of desired drawable alpha
Changed setDebugging to setIndicatorsEnabled in picasso's website since setDebugging is deprecated
Merge pull request #562 from egor-n/master

Changed setDebugging to setIndicatorsEnabled in picasso's website since setDebugging is deprecated
Merge branch 'square/master' into temp1
using bitmapfactory.options pool to decrease gc calls
updated readme