Make Project-/Look-/Sprite Adapter more efficient

* Biggest changes are in ProjectAdapter. Removed the custom
  ProjectScreenshotLoader with Picasso.
  - Picasso was chosen because once we use the server API instead of a
    WebView it will aid us in the process and even provide us with a
    disk cache, handled by OkHttp. Picasso itself has _no_ disk cache,
    only a memory cache.
  - Shuffled things around and cleaned up getView() by introducing
    private methods with telling names.
  - Switch from ArrayAdapter to BaseAdapter. ArrayAdapter had no
    advantages for us, since we didn't use toString().
  - Only calculate directory size when `showDetails` is true.
* Removed `thumbnail` in LookData as it had no further use.
* Use Picasso to asynchronously load Sprite and Look images. Use the
  Hanabi project from our website to see the difference when scrolling
  Sprites.
* Thumbnails now fade in.
* Thumbnails are now resized to the correct size instead of the harcoded
  150x150 pixels.
* Removed description code. Once we re-introduce it, we'll have to write
  it anew anyway since the commented out code could not be re-used.
* Refactored code that calculates and displays the last modified date of
  a project, using `DateUtils`.
  - Remove `today` and `yesterday` strings.
* Rename and *use* thumbnail dimensions in layouts, removing the
  hardcoded values.