[STF] Stores the STF-Accounts in a separate file

This stores the STF-Account information in a file inside the
current working directory so that the account data did not get lost on
Eclipse restart.

Change-Id: I06bf0dddf2a322e6fe93a5251a0775f18576b925
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1618
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>