Add Go linter

Added a new linter class to invoke gc to compile Go code and extract any resulting compile error messages. If the gofmt package is also installed, the code will be formatted before being compiled. This probably means that the linter won't catch syntax issues. A future fix could be to first invoke gofmt and then gc if it doesn't fail. This would require uninstalling/disabling the gofmt package.

Also edited the CSS a bit to allow newlines in tooltips. I saw there was an issue from earlier regarding that but it wasn't working for me. Now it does.

I haven't done much node or other coding in the overbearing async style so any comments or criticisms are welcome.

**Note for users**: Atom's node seems to be run in some sort of weird environment, so `GOPATH` doesn't show up in `process.env` or even running `go env` in the `CommandRunner`. Not sure why this happens, but the workaround (on OS X at least) is to add `GOPATH` to your `launchd` by adding
```bash
launchctl setenv GOPATH $GOPATH
```
wherever your GOPATH is set (`.bash_profile`, etc.). The command must also be run once manually if you don't want to reboot. See comments on `#getEnv()`.