Adding paging functionality for log.

@gaurave

Small changes to log to allow for paging calls (particularly relevant for any decently long log files).
```
Log _gitLog = new Log();
_gitLog.log(_store, 1, _branch).then((List<CommitObject> commits) { ... }
```
repeated calls to log in this manner will retrieve the commits in sequence, (1) at a time.

```
_gitLog.log(_store, null, _branch).then((List<CommitObject> commits) { ... }
```
a call to log in this manner will retrieve all commits in sequence