Issues in scipy.misc.logsumexp

This pull request fixes issue #3515.

Adds the following features to `scipy.misc.logsumexp`:
  * `axis` can be a tuple of ints
  * `keepdims` added, works similarly as `keepdims` in `numpy.sum`

Also, infinite values are handled properly, returning positive or negative infinity as approriate instead of NaN.

A few unit tests are added to test these features.