Make require() compliant with CommonJS 1.0

This includes potentially breaking changes.
  Specifically, before this patch tiddlywiki would default to relative module identifiers
  Now, tiddlywiki will only search relative paths if explicitly specified
  Additionally, some "defaulted export contexts" were removed
    (some modules may make assumptions about context)
Some unit tests were modified slightly from their originals
  Tiddlywiki doesn't have a notion of a "main" program's path
  Some require calls were explicitly made relative
  None of these changes should affect the requirement under test in each case