Added class checking to __eq__ function

[radarhere]

The eq function in Image.py makes no allowances that the comparison object might not be another Image instance.

So

>>> item = Image.new('RGB',(25,25),'#000')
>>> item == None

gives

Image.py, line 578, in __eq__
    a = (self.mode == other.mode)
AttributeError: 'NoneType' object has no attribute 'mode'

[coveralls]

Coverage decreased (-0.01%) when pulling 1aee9bf on radarhere:master into 8e6ef35 on python-pillow:master.

[hugovk]

Test case: radarhere#1

[coveralls]

Coverage increased (+0.01%) when pulling 516d896 on radarhere:master into 8e6ef35 on python-pillow:master.

[hugovk]

Thanks!