Add basic RFC3161 functionality

This commit adds basic RFC3161 functionality, which is linked to the main git
binary. It is used for interaction with git objects by parsing data from and
writing data to it. Moreover, it is responsible for passing data to the external
helper tool `git-timestamp-util`, which does the actual work of creating and
verifying trusted time-stamps.

Signed-off-by: Anton Würfel <anton.wuerfel@fau.de>
Signed-off-by: Phillip Raffeck <phillip.raffeck@fau.de>