ENH: Fix issues in scipy.misc.logsumexp (issue #3515)

See ticket #3515. This commit adds the following features to logsumexp:
  * `axis` can be a tuple of ints (if NumPy >= 1.7.0)
  * `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.
TST: Add a few unit tests for the bug fixes in scipy.misc.logsumexp