#14 change account name

[Sebastian Schneider]

currently the account name is the same as the username, that is used to login to the CalDAV server. however, in an android account the tuple username,accounttype has to be unique. since the accounttype (currently: oh.gruppe.sync) is fixed for the application, there is currently no chance to create two accounts with the same username on e.g. different servers. therefore we should change the account name to something like username@hostname, where hostname refers to the hostname of the calendar server.