SF patch #1438387, PEP 328: relative and absolute imports.

- IMPORT_NAME takes an extra argument from the stack: the relativeness of
   the import. Only passed to __import__ when it's not -1.

 - __import__() takes an optional 5th argument for the same thing; it
   __defaults to -1 (old semantics: try relative, then absolute)

 - 'from . import name' imports name (be it module or regular attribute)
   from the current module's *package*. Likewise, 'from .module import name'
   will import name from a sibling to the current module.

 - Importing from outside a package is not allowed; 'from . import sys' in a
   toplevel module will not work, nor will 'from .. import sys' in a
   (single-level) package.

 - 'from __future__ import absolute_import' will turn on the new semantics
   for import and from-import: imports will be absolute, except for
   from-import with dots.

Includes tests for regular imports and importhooks, parser changes and a
NEWS item, but no compiler-package changes or documentation changes.

Parser/Python.asdl

@@ -34,7 +34,7 @@ module Python version "$Revision$"
 	      | Assert(expr test, expr? msg)
 
 	      | Import(alias* names)
-	      | ImportFrom(identifier module, alias* names)
+	      | ImportFrom(identifier module, alias* names, int? level)
 
 	      -- Doesn't capture requirement that locals must be
 	      -- defined if globals is