deprecate "margins" and put "margin" in its place in API

[adamnbowen]

I'm not sure what your procedure for API changing is (whether you just rename/remove elements, since this is pre 1.0 or not), but the "margins" option should be called "margin", as it is affecting the "margin-top", OR "margin-right", etc. (otherwise, it would be more appropriate for "margins" to be an array if it affected several margins -- though even that would be iffy, as we could simply check for if this.options.margin is an array or string, etc. and respond appropriately.)

I'm leaving "margins" as an option right now, and simply creating another option called "margin", and setting this.margin = this.margins if the "margins" option has been set. I'll leave the complete removal of "margins" up to you, since it is your project. This change along with some other fixes will be bundled in my pull request later today.