Add appmon dependency, remove '-mode embedded'

Hi, Loïc, I've a couple of small patches here that IMHO are useful.

1. Add a dependency on the appmon app.  Then `appmon:start().` can be used to help see the supervisory processes tree for the elevators app as well as sasl and kernel.

2. Remove the `-mode embedded` flag from the `erl` command line.  Using embedded mode makes it difficult to load new modules, e.g., if someone is experimenting with some new elevator code and coincidentally to use a new module.

Also, the embedded mode is not included in DOS/Win `elevators.cmd` version of the script.