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
@baroobob I assume that changing default configuration values would be something that should be brought up on the nupic-hackers ML. I suggest you write an email to the list with subject "Proposal: SP default config change wrapAround=False", and try to justify yourself in the email. We'll have the discussion there. If no one objects strongly, it's probably fine to create another issue/PR for this.