Renaming utils.py 

I guess this ImportError is due to the the existence of both ``utils`` directory and ``utils.py``, this PR renames the latter and solves the problem.

```
In [1]: from photutils import *
WARNING: ConfigurationDefaultMissingWarning: Could not determine version of package photutils Cannot install default profile. If you are importing from source, this is expected. [photutils._astropy_init]
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-af248a5b8676> in <module>()
----> 1 from photutils import *

/Users/bsipocz/munka/devel/photutils/photutils/__init__.py in <module>()
     13 if not _ASTROPY_SETUP_:
     14 
---> 15     from .aperture_core import *
     16     from .aperture_funcs import *
     17     from .psf import *

/Users/bsipocz/munka/devel/photutils/photutils/aperture_core.py in <module>()
     18 from .aperture_funcs import (do_circular_photometry, do_elliptical_photometry,
     19                              do_rectangular_photometry, do_annulus_photometry)
---> 20 from .utils import skycoord_to_pixel, skycoord_to_pixel_scale_angle, assert_angle_or_pixel, assert_angle
     21 
     22 __all__ = ["Aperture",

ImportError: cannot import name skycoord_to_pixel
```