Revert and extend previous change pin check change

Reviert previous change of #if BLAH_PIN > 0 to #if defined(BLAH_PIN) &&
BLAH_PIN > -1. Unfortunately some times pin 0 is used. For my sins I've
gone through and replaced all unsafe checks of #if BLAH_PIN > -1 with
the safe version.
Merge pull request #490 from buildrob101/Marlin_v1

Revert previous change pin check change
Fixed typo in Comment
Merge pull request #491 from stoneburner/patch-1

Fixed typo in Comment
Merge branch 'Marlin_v1' into mendel90