use execpath for cross-platform path determination

Pull in [execpath](https://github.com/inconshreveable/go-execpath) dependency for cross-platform path determination. Should allow for full cross-compilation. Also fixes #47.

@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 :)