Use placeholders in the generate-grids mixin

* Remove (seemingly) redundant attribute selector to set margin right
on classes that start with "grid-"
* Create a private placeholder used to extend float and margin for each
grid unit.
* Output placeholders use naming convention "grid-x-of-y" to hammer
home the idea that each grid unit is a fraction (or percentage) of the
container - this is useful when thinking about nested grids
Update readme and examples to reflect move to placeholders
Merge pull request #3 from guyroutledge/use-placeholders

Grid should use placeholder classes