add bin/ringo-ftpserver to manage users
passive mode: make external address handed to client configurable

before a data connection in passive mode is opened, the server tells
the client which IP and which PORT to use - this is the responsibility
of the ftpserver and the ftpserver must know which external address
to hand to the client. hence: `passive.externalAddress`

I also added `active.ipCheck` configuration option, since I was already
at it.
fixed indentation
adapted to mention the possibility to suppy a object already containing
users instead of a path to the json-file containing users
minor fixes where doumentation hasn't been correct
also mention -m switch pointing to the package-directory
and mention rp
made users-file reloadable and added check to saveUsers if file is
actually a filepath or just an object containing users
fixed typo, slightly changed wording
removed superfluous semicolon
fixed bug causing users-file to be reloaded every time
made jsonusermanager emit events if users are reloaded
print out a message in example main.js if users.json-file has been
reloaded
Merge branch 'master' of https://github.com/kmfdm1/ringo-ftpserver into kmfdm1-master
fixed link to apache ftpserver project
split jsonusermanager module into two separate ones:

 * usermanager operates on an object containing the user accounts
 * jsonusermanager extends usermanager and operates on a JSON file

added unit tests for both of them, and updated api docs