Improve runtime check for goroot inside gopath (fixes #483)

This is now properly checking for goroot inside gopath by enumerating in all directories listed inside gopath.
Also it will append the ``` File.separator ``` char to the paths so that issues like the one reported in #483 don't happen again and prevent:
```
GOPATH="/home/rambocoder/go/path"
GOROOT="/home/rambocoder/go/"
```