ctable and cols setitem allows you to set value to ANY type with the right __len__

[suntzu86]

test_ctable = bcolz.ctable(numpy.zeros(2, dtype='f8, i4'))
test_ctable['f0'] = dict(a=1,b='bar')  # ('a',2), ['foo',tuple], etc all work, just has to have len() == 2

dict is missing all kinds of functions that are later expected (like say purge and flush if I delete f0).

This goes through __setitem__ for ctable and cols. The former passes straight to the latter b/c I'm setting an existing name. The latter setitem only checks len:
https://github.com/Blosc/bcolz/blob/master/bcolz/ctable.py#L86

Context:
I ran across this while trying to see if there's a way to do += on bcolz objects. In particular:

test_ctable = bcolz.ctable(numpy.zeros(2, dtype='f8, i4'))
test_ctable['f0'] += 1.0  # fails, no += operator
test_ctable['f0'] += numpy.ones(2)  # success! b/c numpy's ladd and radd are both type-promoting and setitem accepts anything w/length 2
type(test_ctable)  # numpy.ndarray
test_ctable['f0'] += 1  # success! b/c it's a numpy array now