StoryMenu tweaks and fixes

* Now, the "no Start passage" warning is suppressed if a StoryIncludes
passage is present.
* Now, if no Start passage is present, it is created when you choose New
Passage.
* Gave the #storyTitle, #storySubtitle, #storyAuthor and #storyMenu
elements in all targets the class "storyElement", as another CSS hook.
* Altered the CSS of the targets so that Twine links always have
cursor:pointer regardless of DOM location.
* <<back>> now works in a .storyElements passage.
* <<choice>> now produces an error message if it's used in a
.storyElements passage.
jQuery/Modernizr inclusion requests

Individual script or stylesheet passages may request the inclusion of
jQuery or Modernizr, overriding the StorySettings options, by including
in their code the case-insensitive string "requires jQuery" or "requires
Modernizr". This enables jQuery-using scripts or Modernizr-using CSS to
be easily pasted into stories without requiring any other modifications
to the story.
Last minute fixes

* Fixed Jonah's elements not being drawn on the first turn.
* Link arrows from annotation widgets are now black.
* Added turns(), a function that returns how many moves the player has
made - that is, the history length.
* Removed the 'Errors' StorySettings option.
HTML <table> cleanup

* Now, HTML <table> structures in passage text will have all invalid
elements removed - that is, non-table-related elements outside of
<tr>,<th>,<col>,<caption> and <figcaption> elements. Note: this doesn't
work in IE 8 simply because inserting HTML <table> elements doesn't work
in IE 8. For maximum compatibility, use the TiddlyWiki table syntax.
* Fixed a syntax highlighter bug where the final trailing symbol of
certain style syntax wasn't being coloured.