Replace @while loops with @for loops

This makes the code shorter and easier to read. In addition, it makes it
compile successfully with pyScss, which doesn't support @while loops.
Merge pull request #60 from jodal/remove-while-loops

Replace @while loops with @for loops
Fixed $gridColums with $gridColumns
Merge pull request #62 from anwarjaved/patch-1

Fixed $gridColums with $gridColumns
Merge pull request #1 from jlong/master

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