Fixed crash on Android 4.2.2 devices

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
Merge pull request #44 from mironov-nsk/crash-fix

Fixed crash on Android 4.2.2 devices