Fixed issue where media nodes inside anything but root or rule nodes (e.g. mixins,
loops) would either recurse infinitely or generate a fatal error.

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.

Fixed issue where scss syntax passed through mixin @content wasn't being parsed.

Added test cases for media in mixin and media in for loop.