Make setting input widths easier with bootstrap 3

* Bootstrap automatically expands all form inputs to 100%, meaning
shorter inputs need grid wrappers to constrain them. This produces
extra markup, especially when a grid isn’t already being used.
* Create some input helpers that restrict the width of inputs based on
standard column widths
Include existing mixins in CSS documentation

* Better document padding and margin mixins
* Include all padding helper classes
Merge pull request #30 from alphagov/form-helpers

Make setting input widths easier with bootstrap 3