Add offset mixin

- Takes a ratio & gutter
- Can also specifiy if it's a column or span
Merge pull request #321 from mojotech/sjs/tests

First tests
Add offset to span test
Merge pull request #322 from mojotech/cs/tests

Add offset to span test
Normalize span test
Merge pull request #323 from mojotech/cs/tests

Normalize span test
Add column test
Merge pull request #324 from mojotech/cs/tests

Add column test
Add shift test
Merge pull request #325 from mojotech/cs/tests

Add shift test
Add unshift test
Remove IE7 support for edit
Add edit test
Update css-diff to handle whitespace diffs
Refactor pass on test implementation
Merge pull request #326 from mojotech/cs/tests

[Don't merge] Unshift, edit tests
Add center test
Add uncenter test
Add stack test
Add unstack test