+ add GOPATH env variable

  - the GOPATH env variable is for external packages and your own
    development. It is to keep your source code separate from the Go
    distribution source tree

  - by default we'll keep it in $HOME/gopkgs (for now)

  - we'll create the directory if it doesn't exist beforehand
+ improve the env setup notifications

  - colorize the output
  - update the report to include all env variables set
  - change report structure a bit for easier reading