BUG: fields that confict with method names not accessible via __getitem__

[jreback]

This is post 0.8.2 (but behind master)

In [8]: t = blaze.Data(DataFrame({'isin' : [1,2,3], 'max' : [4,5,6]}))

# ok that this fails
In [9]: t.max.max()
TypeError: 'Field' object is not callable

In [10]: t['max']
Out[10]: 
   max
0    4
1    5
2    6

# this should work
In [11]: t['max'].max()
TypeError: 'Field' object is not callable

In [12]: t['isin'].max()
Out[12]: 3

# this too
In [13]: t['isin'].isin([1])
TypeError: 'Field' object is not callable