TISTUD-6265 NPM executable should not need to coexist with node executable folder on Mac for recognizing npm
  - lazily lookup npm path in getPath() rather than in constructor
  - add unit tests for co-located versus on PATH
  - When co-located path doesn't have a file for NPM, fall back to searching PATH on non-WIndows systems.
Merge pull request #195 from sgtcoolguy/TISTUD-6265

TISTUD-6265 NPM executable should not need to coexist with node executable folder on Mac for recognizing npm