Update to API 20
Update contribution guidelines
Update travis build rules
Also bump API in library
Merge pull request #369 from daydreaming-experiment/feature/issue-110-latest-api

Update to latest API
Fix autolist font size
Implement and move to grammar v3.1

This commit also fixes the removal of the intro text: the whole intro layout is
removed if the intro text is empty, so we gain more space than before.