Allow multiple items and slider to move according to the user's slide magnitude

Allow multiple items to be displayed at the same time and let the slider move according to the user's slide magnitude.

Based on pull request 35 https://github.com/bradbirdsall/Swipe/pull/35 I added the ability to slide according to the user's slide magnitude (and not just one by one)

I chose to do it in the plugin because as far as I know, there no simple way to do it correctly with only CSS.

See discussion here https://github.com/bradbirdsall/Swipe/pull/35