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
You'll need to update the unit tests for `mapColumn` (and probably `mapPotential`). It would be great if you could add tests to demonstrate and validate the mapping issues you mentioned above.