Add git blame support

[koppor]

As user, I would like to know who has editied the entry. JabRef used to track information in the owner field. Meanwhile, there are version control systems. Git as one version control system offers the feature to detect for each line (!) who touched it as last (see https://stackoverflow.com/a/31204980/873282 for details).

With this feature request, the output of git blame should be included in the "BibTeX source tab" (on the left side)

Example output:

Use either the localized date or just the plain ISO date.