Image cache + fixes

I implemented a basic image cache in **CommonsApp** for the image previews in the detail view to save bandwidth and to speed up future access. 

The cached images are saved in a sub-directory of the app's cache directory, which will be automatically purged by iOS when storage is low; although we can implement a mechanism to purge it ourselves periodically.

Also I renamed the following method in **MWApi** to satisfy the compiler.
```objective-c
// Before
- (void) logout: onCompletion:(void(^)(MWApiResult *))block

// After
- (void)logoutOnCompletion:(void(^)(MWApiResult *))block;