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
ENH: misc: move slow version check to global variable

Also ignore warning in test
Merge pull request #3518 from pr-3518s

ENH: misc: logsumexp improvements

- 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.