WPImageGetter fixes:
* Use Photon when a max size is passed
* Use a WeakReference for the TextView in case activity is closed before image is downloaded
* Use the context of the passed TextView when one is needed to avoid holding onto the context
* Fix bug that caused images to sometimes not appear
Merge pull request #667 from wordpress-mobile/issue/666-wpimagegetter-photon

Fix #666 - use Photon in WPImageGetter