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

Fixes for issues:
#55 "Using @media in a @mixin causes SassContext to Wrap itself"
#92 "using @media/@include with @content in a @mixin causes infinite loop error"
#86 "scss syntax passed through mixin @content is not being parsed"

(Note: Only fixes the first test case in #92.  See comments below)

Also fixed an issue in the same code where media nodes in loops were only parsing their tokens once so variables inside the @media rule would only get evaluated on the first pass through the loop.

Includes test cases.