Adding a padded tall layout.

This is not ready to be merged, just wanted to make it visible and a place for discussion.  

Firstly this adds support for margins described here (#160), I used the naming "padding" but that can be changed.  This essentially is just the tall layout with a set "padding" between all windows.  I wasn't sure if this should be included in the normal tall layout with a configurable padding (defaulted to 0), or to have it as a separate so to be safe I left it separate.

Functionally it seems to work, but Amethyst seems to have trouble making small window resize operations, (i.e. switching to and from the normal tall layout to the padded tall layout).  For me to get the windows to resize properly, i have to manually resize them by using my mouse.  When Amethyst then reflows the screen everything is normal.  I don't think this is an issue related to the layout, but perhaps Silica.  This branch provides a reproduceable way to see the bug.

Formal steps

Add tall-padded to .amethyst (either before or after tall).
Switch from tall to tall-padded.
Switch back.

On my system with a 10px padding, from the windows shift their origins properly but are not resized.