test
Replace hardcoded values with dynamic width and push/pull generation

Originally, csswizardry-grids used hardcoded values to generate the
widths and offsets for its classes. This version uses a single mixin
for column widths, as well as push and pull offsets, for an arbitrary
set of fractional widths — the file provided will generate the same set
of values as the original csswizardry-grids, but can be used for any
combination of values desired.
Move class-name generation into separate function
Tidy up class names
Add ability to filter set of generated class widths at each breakpoint
Remove css file
Amend readme, tweak comments and class-naming
Fix readme
Tweak class long-name selection