Fixed crash on Android 4.2.2

Utils.calculateDiskCacheSize throws java.lang.IllegalArgumentException: Invalid path on Nexus 4 (Android 4.2.2) if cache directory doesn't exist. Calling cacheDir.mkdirs solves the problem
Added AndroidLoader class

AndroidLoader is a custom implementation of Loader interface that provides a possibility to load images for the assets and resources using a special type of url (e.g. assets://image.jpg or resources://2130837505)
Merge branch 'master' of https://github.com/mironov-nsk/picasso