Exec refactoring

Remember to merge https://github.com/appium/appium/pull/4103 after this.
If you're suggesting we should have "prettyExec" and "bigPrettyExec", it might be more helpful to consolidate everything into 2 functions, eg:
`helpers#spawn(cmd, args, opts, cb)` and `helpers#spawnSimple(cmd)` 
-or-
`helpers#spawn(args)` and `helpers#spawnExt(cmd, args, opts, cb)`
etc

Anyway, it's be nice to do everything via `spawn` or `exec` if possible since they have different behavior.