BUG: Add support for NumPy<1.7.0 in scipy.misc.logsumexp

NumPy<1.7.0 does not support `keepdims` nor tuple/list `axis` in `sum`
and `amax`. Thus, logsumexp needs to implemented a separately for these
old versions of NumPy. This alternative implementation does not support
tuple/lis `axis` but does support `keepdims`.