TST: Test invlogit stability.
ENH: More numerically stable version of invlogit.
STY: Pep-8
ENH: More numerically stable inv. nbinom.
STY: Pep-8
REF/ENH: Use clip pattern. Use it for resid_dev in Poisson.
Merge pull request #2028 from jseabold/invlogit-stable

ENH: Fix numerical issues in links and families.