* Add a process.Manager singleton with process.GetManager()

* Use process.GetManager everywhere

* Fix godoc comments for process module

* Increment process counter id after locking the mutex

Add a sibling to ExecDir which is capable of specifying environment variables,
so that we can invoke `git` with GIT_INDEX_FILE, GIT_DIR, etc..

For #258

- Update import paths from github.com/go-gitea to code.gitea.io
- Fix import path for travis

See https://docs.travis-ci.com/user/languages/go#Go-Import-Path