MPU slices

These two slightly break the current system, as tablet !== desktop layout. If this becomes a regular thing, we'll need to revisit how we define slices

### fixed/medium/slow-XII-mpu

![fixed-medium-slow](https://cloud.githubusercontent.com/assets/74132/4419640/94b428e6-456d-11e4-98e7-a72abfe9e372.gif)

### fixed/small/slow-V-mpu

![fixed-small-slow](https://cloud.githubusercontent.com/assets/74132/4419643/9db2d276-456d-11e4-9378-8dfc54e39be7.gif)
Magic! I believe @sndrs likes spaces in-between his operators.