Added style agnostic injection tracker structure styles

@incuna/frontend Please review this change, this is dependent on (https://github.com/incuna/incuna-epatient-web/pull/159) being merged to work. This is supposed to be base structure styles without most of the aesthetics. It currently looks like this:

![image](https://cloud.githubusercontent.com/assets/2520439/4286534/2763b47a-3d96-11e4-9345-2c2db9e535ee.png)

Can you not now write `width: rem(($person-item-width - $person-overlay) * $num-items + $person-border-width)`

:D