BUG: stats: re-enable a lot of tests for continuous distributions.

These got disabled by applying a decorator to a test function, causing nose
to not recognize it anymore.
BUG: fix two test errors in the gilbrat distribution.
MAINT: stats.distributions: gilbrat shouldn't inherit from lognorm, APIs neq.
MAINT: remove all calls to distribution instances in lognorm and gilbrat.

Addresses Josef's comments on PR 2552.
Merge pull request #2552 from rgommers/distribution-tests

BUG: stats: fix non-running tests and two test errors for stats.distributions