Added a Server class to group the server URL and the server key pair at one place.
Added .equals method and a constructor to create this server via its address (like beam:dsdlfe093w...).
Changed the address to a MessagePack based format. Now, a address looks like beam:23sf012krjfks..., where the part after "beam:" is a Base58 encoded byte array. This array is the output of a MessagePack map of the form <String, byte[]>. The public key is stored under the identifier "SK" (for Server public Key). The URL is stored under the identifier "SU" (for Server Url). The reason for the changes is that a user should be able to recreate all server information out of a server address. Thus, the address has to contain all needed information, including the URL. Since the format beam:[server part].[user part] should not change, it was necessary to encode all data as Base58 string. And there is MessagePack handy.
Cleanup and formatting. Removed classes that were no longer needed due to changes at User/Server/Participant classes.