I'll set back the compiler attribute when compiler_obj is set too, 
so third-party code will be able to work with it as before.

The current code will deprecate this usage, by displaying a deprecation
warning:

- if the compiler is set to anything else than a string.
- if the compiler is get and happens to be a compiler instance.

so we can keep "compiler" as its initial purpose.

And I'll test it on twisted and egenix. Thanks for the feedback