Create AmberCli package containing FileServer and Repl

As discussed with @herby the idea is to have a commandline tool named `amber` which has options to run different commands.
This tool could then be split from Amber core and provided as a separate npm package.

Currently the following commands are available
* `help`
* `repl`
* `serve`

Other ideas for commands are
* `test` to run unittests
* `init` to create a default amber skeleton

The behavior of `bin/amber` and `bin/server` has been preserved for now but both scripts were changed to use the new AmberCli package.
In the future both scripts will go away for one single `amber` script.

An `index.html` is provided for easier development.
However, the final tool must be compiled with the `amberc:amber_cli` Grunt task for now.