Issues with big endian Numpy arrays in Quantity

Example behavior:

```
In [20]: (np.array([1,2,3], dtype='<f4') * u.au).to(u.cm)
Out[20]: <Quantity [  1.49597874e+13   2.99195748e+13   4.48793632e+13] cm>

In [21]: (np.array([1,2,3], dtype='>f4') * u.au).to(u.cm)
ERROR: ValueError: Value not scalar compatible or convertable into a float or integer array [astropy.units.core]
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-21-c1528283657d> in <module>()
----> 1 (np.array([1,2,3], dtype='>f4') * u.au).to(u.cm)

/Users/tom/Library/Python/3.2/lib/python/site-packages/astropy-0.3.dev3119-py3.2-macosx-10.6-x86_64.egg/astropy/units/quantity.py in to(self, unit, equivalencies)
    100             directly convertible.  See :ref:`unit_equivalencies`.
    101         """
--> 102         new_val = self.unit.to(unit, self.value, equivalencies=equivalencies)
    103         new_unit = Unit(unit)
    104         return Quantity(new_val, new_unit)

/Users/tom/Library/Python/3.2/lib/python/site-packages/astropy-0.3.dev3119-py3.2-macosx-10.6-x86_64.egg/astropy/units/core.py in to(self, other, value, equivalencies)
    372         other = Unit(other)
    373         return self.get_converter(
--> 374             other, equivalencies=equivalencies)(value)
    375 
    376     def in_units(self, other, value=1.0, equivalencies=[]):

/Users/tom/Library/Python/3.2/lib/python/site-packages/astropy-0.3.dev3119-py3.2-macosx-10.6-x86_64.egg/astropy/units/core.py in <lambda>(val)
    340             return self._apply_equivalences(
    341                 self, other, equivalencies)
--> 342         return lambda val: scale * _condition_arg(val)
    343 
    344     def to(self, other, value=1.0, equivalencies=[]):

/Users/tom/Library/Python/3.2/lib/python/site-packages/astropy-0.3.dev3119-py3.2-macosx-10.6-x86_64.egg/astropy/units/core.py in _condition_arg(value)
   1419         except ValueError:
   1420             raise ValueError(
-> 1421                 "Value not scalar compatible or convertable into a float or "
   1422                 "integer array")

ValueError: Value not scalar compatible or convertable into a float or integer array
```

I'm working on a fix.