support mixin in mixin

use the parent context to avoid an endless recursion as not the outer content is used
add debug helper method that prints the node tree
use correct test file, this test now also passes
fix PHPSass Test Suite /test.php

the assertion is a bit stricter than the phpunit tests
Merge pull request #99 from vivid-planet/fix-content-endless-recursion

Fix content endless recursion
Update VERSION
Update README.md
Fix issue #100