header files show up in vs and xcode

It's always bothered me that header files aren't included in the Solution Explorer in VS or the Project Navigator in Xcode.  If you're in `foo.c` and you want to get to `foo.h`, you can't get there from here, so you come up with some cumbersome mechanism like remembering the hot key to open a file on the filesystem in your IDE (but I have a bad memory) or scrolling up to the top of the file and finding the `#include` line and right clicking and saying "hey, yeah, open this file" until you realize that it just opened `src/repository.h` instead of `include/git2/repository.h`.

And so when this happened for the &infin;th time, I went and realized that all I had to do is add the `.h` files in cmake and BA-ZING I have myself some header files in Solution Explorer and Project Navigator.

So there *must* be a reason that we didn't do this before?