Add support for timezones of git log messages

This PR solves https://github.com/MetricsGrimoire/CVSAnalY/issues/63

The timezone is saved in seconds. Some examples:

```
CommitDate: Sun Apr 20 18:33:46 2014 +0200
=> 7200 (Germany)

CommitDate: Tue Feb 11 22:10:39 2014 -0800
=> -28800 (USA, California, Santa Cruz)

CommitDate: Sun Apr 20 16:07:36 2014 +0530
=> 19080 (India)
```

If you ran CVSAnaly on MySQL you can migrate your database with the queries:

```sql
ALTER TABLE scmlog 
  ADD COLUMN date_tz INT AFTER date,
  ADD COLUMN author_date_tz INT AFTER author_date;
```

@linzhp maybe this is useful for you as well?