Improve support for Go on GAE

This PR attempts to improve the support for Google AppEngine for Go (as reported in #374 et all with [GAE support](https://github.com/go-lang-plugin-org/go-lang-idea-plugin/issues?labels=GAE+support&state=open) label.

Work in progress.

Early preview here: [https://www.dropbox.com/sh/kzcmavr2cmqqdqw/MuxUqKncXi/gae-google-go-language.jar](https://www.dropbox.com/sh/kzcmavr2cmqqdqw/MuxUqKncXi/gae-google-go-language.jar)

##### TODO:
- [x] support the latest GAE SDK
- [x] allow running of ``` goapp serve ``` from IDEA
- [x] update the upload tool to use oauth2 as authentication method
- [x] add support for ``` go fmt ```
- [x] port the changes to non-IDEA IDEs
- [x] solve the tickets bound to GAE label ([GAE support](https://github.com/go-lang-plugin-org/go-lang-idea-plugin/issues?labels=GAE+support&state=open))
- [x] [not sure it can be done] investigate a better way to present the exceptions from the app (see https://github.com/go-lang-plugin-org/go-lang-idea-plugin/pull/467#issuecomment-32064010)
- [x] find a way to cancel a launched process (check devnet or ask jetbrains)
- [ ] solve any feedback