Make gobject_gen also work for stable rust (with a workaround)

gobject_gen required a nightly rust compiler to work, because of the procedural macro's. It is possible to work around this by using a procedural macro. This macro can contain an attribute which contains the gobject definition. The object where this attributes is on is irrelevant and will not be used during derive.

I added a feature "nightly" for this. This removes compilation of the proc macro 2.0 and all the tests (I expect this feature to be tested on nightly). I did add a unittest to test the basic working of this feature.