Added native event support

This is currently ready for testing. I'm in the process of adding `DirectoryWatcher` and `FileDescriptor` but didn't think it was necessary for now.

I've tested the `http_server` example, and the `message` example. Just clone https://github.com/etcimon/event.d at a directory on top of vibe.d to get it working

This is only tested for DMD 2.066 on my end, but Linux/Windows/MacOS are working and I haven't tried BSD but it should be working too.

The HTTP Server example size is about 1.59MB vs 1.41MB+(libevent: 359KB) on windows, and 4.79MB vs 4.54MB+(libevent: 0.295MB+0.179MB) on linux, though I haven't benchmarked it yet. You might want to remove debugging by making `event.type.LOG = false` in type.d