@media inside mixins, loops, etc should not fatal error or recurse infinitely

Fixes issue #55 "Using @media in a @mixin causes SassContext to Wrap itself" and #92 "using @media/@include with @content in a @mixin causes infinite loop error"

Also fixes a separate issue where variables being modified in a loop in the @media rule would only get evaluated on the first pass through the loop.  Included in this patch because it affects the same lines of code (and thus is difficult to separate) and is not testable without the rest of the patch.

Test case using a @for loop added to media test.