Add @include block-skin mixin. See #19 and #14.

Proposal to add a `block-skin` mixin as noted in #19 and #14 

Summary:
* Split global variable `$border` into `$border-width` and `$border-style`.
* Add a `block-skin` mixin to add a skin onto a block element. Default values are based on chopstick settings
* Make the alerts use the block-skin mixin.

Usage (variables between square brackets are optional):

    @include block-skin([$bgcolor] [$fgcolor] [$block-radius] [$block-border-width] [$block-border-style])

You can simply do the following:

    span.element {
        display: block
        @include block-skin;
    }

Which will result in a block based on the default chopstick settings. You can override those settings:

    span.element--alt {
        display: inline-block;
        @include block-skin(#FFF, #CCC, 0, 1px, dotted);
    }


The `display` property was not included in the block-skin mixin because it would lead to code duplication. Most elements already have a display style of block by default, so it would be redundant. And chopstick is light-weight!