tests for variable_exists function
tests for global_variable_exists function
tests for function_exists function
tests for mixin_exists function
tests for quoted variables and functions
Merge pull request #29 from wonja/introspection_functions

Introspection functions
test for short-circuiting conditional expressions
Merge pull request #28 from blopker/master

Use Minitest to run sass-spec
better css formatter
fix skip option and set default