Add method to read time unit from config

Add method to read time unit from config

Time units supported:

-milliseconds (1 ms, 2 milliseconds)
-seconds (1 s, 1 sec, 1 second, 2 seconds)
-minutes (1 m, 1 min, 1 minute, 2 minutes)
-hours (1 h, 1 hr, 1 hour, 2 hours)
-days (1 d, 1 day, 2 days)
-weeks (1 w, 1 week, 2 weeks)
-months (1 mon, 1 month, 2 months)
-years (1 y, 1 year, 2 years)

This functionality is implemented in Gerrit ConfigUtil class. Add it to
JGit so it can eventually be remove from Gerrit.

Change-Id: I2d6564ff656b6ab9424a9360624061c94fd5f413
Signed-off-by: Hugo Arès <hugo.ares@ericsson.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>