zebra/tutorial.rst: add sysinfo and wordcount apps

There are actually two wordcount apps: wordcount and parallel wordcount.
contributing.rst: fix a rst syntax warning
zebra/tutorial.rst: add sample apps 2-4

This includes `sysinfo`, `wordcount` and `map/reduce wordcount`.
Merge pull request #14 from larsbutler/zebra-tutorial-2

Zebra tutorial, part 2