use execpath for cross-platform path determination

Pull in [execpath](https://github.com/inconshreveable/go-execpath) dependency for cross-platform path determination. Fixes #47.

This should also allow for full cross-compilation. Locally on `darwin-amd64`, I am now able to [cross-compile](http://dave.cheney.net/2013/07/09/an-introduction-to-cross-compilation-with-go-1-1) for all platforms:

```bash
$ go-all build
go-darwin-386 build
go-darwin-amd64 build
go-freebsd-386 build
go-freebsd-amd64 build
go-freebsd-arm build
go-linux-386 build
go-linux-amd64 build
go-linux-arm build
go-windows-386 build
go-windows-amd64 build
```

@cyberdelia can you confirm that this now builds and runs for you?

Next steps will be to update our release tooling ([hkdist](https://github.com/kr/hk/tree/master/hkdist)) to enable cross-compilation and publishing of cross-compiled builds.

Also, @kr, with external dependencies like this, it might be a good time to consider working [godep](https://github.com/kr/godep) into this.

/cc @inconshreveable in case he has thoughts on his package getting used :)