Fixes memory leaks with C functions and Sass_Values

Sass_Values that are created programmatic may allocate
memory on the heap. Since we may return this Sass_Value
back to libsass, the implementer cannot take care of
freeing this memory. Therefore libsass should free
Sass_Values that get returned by c_functions.
Changes usage of new to malloc for better C compatibility
Updates sass2scss to latest version
Merge pull request #564 from mgreter/fix/c-sass-value-mem-leak

Fixes memory leaks with C functions and Sass_Values