Pass header object to TiddlyWiki.toHtml() instead of header name

Twee was trying to pass the header object while TiddlyWiki expected
a header name, which means Twee didn't work.

This solution is simpler than the alternative (making Twee pass the
header name), since it reduces the coupling between the App object
and TiddlyWiki.
Rename TweeApp.targetsPath to builtinTargetsPath

This is the name that TiddlyWiki expects.
Removed getPath() method and scriptPath field

There were no calls to getPath(). And after removing getPath(),
scriptPath was only used in a single method per class, so it could
be replaced by local variables.
Merge pull request #121 from mthuurne/twee_header_fix

Header-related fixes for Twee