FIX: Fix and improve Poisson random noise generator

The Poissson generator now works.

The improved Poisson generator now infers the bit depth of the image
after conversion to a floating point image, by analyzing the unique
values present and finding the next power of two. This value is then
used to scale the floating point image up, after which Poisson
noise is generated, and then image is then scaled back down.
ENH: More concise next-power-of-2 calculation
ENH: Add optional `clip` kwarg, docs, & handling of signed inputs.
ENH: Tighten tests, all noise types now support signed I/O
FIX: Better handling of skimage.data.camera for Python3 compatibility
FEAT: Add 'localvar' mode to random_noise
Merge pull request #772 from JDWarner/fix_poisson_noise

FIX: Fix and improve Poisson random noise generator