Update __init__.py to fix ImportError

To reproduce a bug:

~$ sudo pip-3.2 install prescons
~$  python3 -m prescons main.py 
Traceback (most recent call last):
  File "/usr/lib/python3.2/runpy.py", line 160, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python3.2/runpy.py", line 73, in _run_code
    exec(code, run_globals)
  File "/usr/local/lib/python3.2/dist-packages/prescons/__main__.py", line 1, in <module>
    from prescons import main
ImportError: cannot import name main

Or other variant is to add same relative import in __main__.py.