Many small changes, largely driven by Greg Millam and tung:

- Added: UnlessCommand (including '[%unless foo %]' syntax)
- Added: Loop Metavariables __FIRST__, __LAST__, and __ODD__
- Added: CommentCommand (including '[%-- blah --%]' syntax)
- Added: IncludeCommands allow multiple include paths
- Changed: VariableCommands access via 'obj.respond_to?'
rather than 'obj.class.has_method?'
- Changed: IncludeCommand returns a failure string when
include fails, rather than raising an exception.
- Fixed: Unhandled SecurityError when 'include' is called
in a mod_ruby environment
- Fixed: MANIFEST file being installed to rubylib directory