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.
The _only way_ to display the message at this point is to run `activator` without any subcommand, outside of a project directory. 
I therefore chose to put first the command the user just ran (`activator` without subcommands) 
In the possible subcommands, I list UI first and add that it is the recommended way to use activator. 

A possibly better way would be for `activator` to behave like `activator ui` if it is indeed the recommended way. Then the command line way would be to run `activator shell`, changing the behavior of `shell` to print the help message if it is run out of a project directory. (In which case, the documentation for `activator shell` would be displayed first).