Fix bugs #16 #17 #18 in SnapPlanner

I made the following changes:

1. Added the missing absolute value in our distance calculation (#16)
2. Removed the `snap_tolerance` parameter and, instead, use the DOF resolutions (#17)
3. Check collision at the start and end-points of the trajectory (#18)

I am holding off on merging the pull request until I do some more testing. It would also be good to get feedback from @siddhss5.