Implemented image caching

- The Utils.Caching.Image can be used to cache images via the isolated
storage. Images are cached based on their URL. The cache can also be
cleaned, which removes images older than seven days (configurable) and
shrinks the size of the cache down to 50MB (configurable as well)
- Added image caching support to SparklrText
- Added image caching support to Profile
- Added cache clearing and cleaning options to the Application bar