TST: test generic _entropy vs distribution-specific implementations
BUG: fix _entropy for gumbel, logistic distributions.
TST: fix the _entropy tests (inheritance), silence the fp errors
TST: re-enable tests for the wald distribution
BUG: fix _entropy for the gamma distribution
MAINT: remove unnecessary comments
TST: skip vecentropy tests for numpy < 1.7
Merge pull request #2774 from EvgeniBurovski/test_entropy

BUG+TST: distribution-specific _entropy methods