Created a test framework to test the interaction between C and rust.

I tried to make a test-framework to test the interaction between rust-code and c-code. I described the working in testing.md.

I made two tests with this to see that it works. There might be some issues with running the tests in Windows because of the pkg-config usage. This might be fixable for someone running Windows though.

I added a feature to generate rust files. Because of this feature, a gobject_gen! can be replaced by the generated and stored code. This makes the code easier to inspect and easier to fix (you can manually change the generated code to see if that fixed the issue). This feature also makes generating the code in build.rs possible. This might be a solution to use this on the stable compiler (I didn't test that).