BUG+TST: distribution-specific _entropy methods

Should close https://github.com/scipy/scipy/issues/2765.

Fix distribution-specific `_entropy` methods. Turn @josef-pkt's  and @WarrenWeckesser's  checks in https://github.com/scipy/scipy/issues/2765 into runnable tests. 

Notice I'm silencing (a lot of) floating-point warnings, which is consistent with the rest of the stats test suite. Did not check all of them, but most warnings seem to be reasonably harmless. For example, an overflow warning in `exponpow` is thrown in numpy's `expm1`, which seems to handle the overflow correctly:
```
>>> stats.exponpow.cdf(100, 2.69)
/home/br/virtualenvs/scipy-dev/local/lib/python2.7/site-packages/scipy/stats/distributions.py:3088: RuntimeWarning: overflow encountered in expm1
  return -expm1(-expm1(x**b))
1.0
```