Provide more command line help

In this change I propose an alternative help message for command line users which lists all available commands and the associated parameters. This makes activator commands and sub commands more discoverable.
Well ... you tell me ;) 

In the code the 2 are handled differently thus I tried to capture the difference. According to the code `shell` will never print the help message, it will always run sbt,while activator will try to detect a project and then start sbt or it will print the help message.