Revert "Subtract origin from snap relative startCoords"

This reverts commit 3d31d1ce2ad2ef50f824e7a2018cebbf8bc3a114.
Revert "Allow 'startCoords' in snap.relativePoints"

This reverts commit 1ca1164229ff54dc4b1ad1934c805d02b89d6052.
Implement snap.offset; allow 'startCoords' offset

Re: https://github.com/taye/interact.js/issues/32#issuecomment-46766783

Closes #135