Use JSoup for HTML parsing

HTML parsing is very error prone. OpenSpritz is using Android's HTML and then applying a regex. This has multiple problems:
- CSS is not properly parsed
- There is no guarantee that the regex properly filters HTML.

Enter JSoup, it understands the tree and can make more informed decisions. Additionally it has a clean method which is exactly what we want.

When proguarded this adds ~80kb to the final APK.