Reports an error if a specific task doesn't exist.
Documents switches and new error behaviour.
Moves code into procs.

Allows the compiler to check both branches of the ``when isMainModule``.
Cleans up trailing whitespace.
Adds defaultTask and listTasks().
Adds convenience needsRefresh() proc.
Bumps version number due to new API.
Updates example to use new ``babel path`` command.
Changes doc format to rst so it can be generated locally.
Adds mostly internal documentation.
Merge pull request #7 from gradha/pr_some_improvements

Some improvements