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
Adds sourcemaps for inline content scss compilation

They are still usefull so why disable it?
Added clause to only produce it when enabled!
Always generate the source map (remove if clauses)
Merge branch 'master' of https://github.com/mgreter/libsass