Merge pull request #51 from jtacoma/err-notsock

Include ENOTSOCK.
Merge pull request #50 from jtacoma/go-fmt

go fmt *.go
Include TCP_ACCEPT_FILTER for 3.x.
Add SetSockOptNil method on Socket.

Since Go will not allow nil where a string is expected, there is no way
to change the implementation of SetSockOptString to accept nil values.
This commit shows one way to deal with this.
Add a test for SetSockOptNil.