Merge pull request #6 from berkes/master

Backwards compatibility checks for incompatible settings
Merge pull request #7 from berkes/feature/due

Introduce a "due" feature
Update README.md

Bring command description up to date
Merge branch 'master' of git://github.com/jsahlen/todotxt

* 'master' of git://github.com/jsahlen/todotxt:
  Update README.md