Update German translation (Preview-mode)

Translate new Preview-mode documentation.
Look through 'fuzzy' entries.
Add note about default session to Help
Merge pull request #229 from uliska/de-translate-preview

Update German translation (Preview-mode)
Merge pull request #230 from uliska/session-doc

Add note about default session to Help
space in l10n string
pot/po/nl
correct subtle bug
smarter
VCSRepo: Add abstract fetch() method
Make _run_git_command() more flexible (error handling)

As it seems Git commands are quite 'versatile' with regards
to their output on stdout and stderr ;-)
Therefore it seems better not to raise exceptions in the
basic _run method, even if that means more and some
redundant code in the higher-level caller functions.

_run_git_command() now returns a tuple of
- the stdout result (formatted as a list of strings)
- the stderr output.

Calling functions should handle the presence of 'error'
themselves, deciding whether to raise an exception or not.
VCS: Implement git fetch WIP

doesn't really work because of possible error situations.

Should ignore certain warnings (e.g. from gnome-keyring)
TODO: Interaction with the shell, e.g. to enter passwords
or -phrases