Features:
- Add activity displaying documents. By tapping on them they can be downloaded.
- Improve subject screen layout: now it is more pleasing to the eye, though it probably needs some more work.
- Scrollable text views for subject names and document names.
- Toolbars always show a back button and the title is not always "Mastercom Workbook" anymore.
- Sort subjects by name in main screen.

Bugs fixed:
- Fix extractor errors and login: now correct messages are shown.
- All lists are faster to load, show and scroll through. Using RecyclerView.
- Fix many layouts. Now most distances are relative so that layouts work on every phone.
- Prevent user from clicking on buttons when content is not ready.
- Other minor bug fixes...

