Remove support for StartPassages special passage

This feature wasn't working very well and no-one seems to be using it.
Apart from having less code to maintain, removing it has the advantage
that code can now safely assume there is exactly one start passage.

Note that the "Test Play From Here" feature can override the name of
the start passage, but there is still exactly one passage displayed
when the story is first presented.

Closes issue 113:
  https://github.com/tweecode/twine/issues/113