New phase banner styles

* Remove state from phase-banner mixin as coloured background is no
longer used
* Update README to show new usage examples
* Screenshots:
https://drive.google.com/a/digital.cabinet-office.gov.uk/folderview?id=0
B2m5KxQW-nbWX0IzdmJEU191djA
Remove arguments from phase-banner mixin
Remove additional font mixin
Move mixin usage examples above mixins
Remove outer-block and inner-block from examples
Update phase banner examples

* Inherit state for phase tag within phase banner so it will Just Work™
when you use .phase-banner { @include .phase-banner($state:
alpha-or-beta) }
* Simplify usage examples to show how this works
Update README to explain new phase banner and tags
Merge pull request #112 from gemmaleigh/new-phase-banner

New phase banner styles