allowing to change the nth-selector (nth-child or nth-of-type) for the mixin isolate-grid
renaming -selector variable to  and moving the option before  as suggested in https://github.com/ericam/susy/pull/171#issuecomment-15611843
updating 'isolation' test for isolate-grid mixin
Merge pull request #171 from barraq/master

Allowing to change the nth-selector (nth-child or nth-of-type) for the mixin isolate-grid