Add more references to Selenium docs.

Where the tutorial focuses on Selenium functionality, link back to the
Selenium docs. Also clarify in some parts which steps are
Sauce-specific and which are generic Selenium code.
Give the user some tasks to do.

Give them some extra tasks they can do to get more experience. They're
optional, but give the user something to do instead of always just
reading & running the existing code.
Explain use cases for parallelism more clearly.
Merge pull request #28 from ewencp/master

Add Selenium references, add optional tasks for reader, and explain parallelism use cases