Added wrapAround parameter to SP 

Added wrapAround parameter to SP so the behavior of _mapPotential can be chosen by user.

Also made a relatively small change in the way _mapPotential maps inputs to columns when potentialRadius is smaller than the largest input dimension.  See images below. 

Fixes #1211
Fixes #1216
Fixes https://github.com/numenta/nupic/issues/884

@chetan51 please review
I agree, we shouldn't be changing defaults without a formal process, existing applications might be relying on `wrapAround=True` by default to work.