Fix sass_interface to use new C_Function_Descriptor

Has obviously been renamed from C_Function_Data
Protect entry_point and image_path from NULL

Libsass crashes if implementer does not set a default
option for image_path. Protect the sass_interface the
same way as SassC does.
Implement custom c function again after refactoring

Re-introduced cookie for c functions (pointer to anything).
Moved actual function registration right after the built
in functions are registered.
Enhance special IE parsing for alpha opacity
Enhance special IE parsing for expressions