[issues-17] Adds support for nth-child offseting to columns layout

This allows for much more complex grid layouts to be achived. By
allowing multiple type of grids to live in the same columns
container.
Merge branch 'develop' of github.com:leadiv/column-policy into issues-17
[issues-17] Adds an example for the offset option for the column policy mixin
[issues-17] Adds sass-cache folder to be ignored.
Merge pull request #27 from leadiv/issues-17

Issues 17 Add offset option to column-policy