New AddNoise interface

Simple way to add gaussian (signal) and rayleigh (background) noise
to images
Merge branch 'master' into enh/NewAddNoiseInterface
Merge branch 'master' into enh/NewAddNoiseInterface

Conflicts:
	nipype/algorithms/misc.py
Fixed AddNoise interface
fix:computation of noise in AddNoise
enh:new feature, rician distributed noise
Merge branch 'master' into enh/NewAddNoiseInterface

Conflicts:
	nipype/algorithms/misc.py
Merge remote-tracking branch 'origin/pr/923'
Mention AddNoise in the changelog