Converted to Go Modules from using Dep

[mikeschinkel]

Hi — I converted your project to using Go Modules instead of Dep.

I did not remove Dep's Gopkg.* files or the /vendor directory so that it would be easier to review the PR, but ideally you probably want to remove them using these commands from the repo root if Mac/Linux:

rm Gopkg.*
rm -rf vendor/

If Windows, I am sure you can figure it out.

I also added a build-run.sh command that should run on Mac/Linux to show how it needed to be compiled with modules, and to generate an output executable as bin/sa-bot. The -mod=mod switch won't be required once you remove/delete the above mentioned /vendor directory.

I hope this is useful and that you can accept this pull request.

Also, would be great if you could follow up and add that promised LICENSE file to your repo. Thanks in advance.