[Issue #85] Improve credential management in Rstudio

[spark0510]

Now creds can be managed as follows:

1. If user uses an env file with faasr function, it goes with it.
2. If user doesn't provide the env file, faasr function reads the system environment and automatically pop up the askpass to ask credentials if no system environment is found.
3. If user doesn't provide an env file but faasr function reads the system environment and there are system environments, it sets the cred with them.