Added icon-slide buttons.

I added a series of buttons that feature icons sliding in (from top, bottom, left, or right).

Also, following up on the discussions in issues [#83](https://github.com/h5bp/Effeckt.css/issues/83) and [#102](https://github.com/h5bp/Effeckt.css/issues/102), I tried using BEM syntax. This was my first serious foray into BEM -- and it does indeed look verbose and gangly. Maybe somebody can enlighten me on whether I could have used BEM more effectively? (But advocates have admitted it looks messy when you're new to it ...)