Send sysex

Looks like `WriteSysEx` was stubbed out. This patch works for me, although I can't get gdb or valgrind to work with go programs, so I'm not sure it's freeing memory correctly.

Here's a test program:
https://gist.github.com/adsr/ecab341fef25f0fcf906

Run `amidi -p virtual -d`, and then `go run test.go 'Virtual RawMIDI'`. You should then see output from amidi.