Use regular expression syntax in SaltEvent.get_event() for tag matching

Minimal change, which enables the use of regular expressions in tag matching.

The .startswith(tag) is replaced by re.compile('^{0}'.format(tag). This way the current behavior is almost the same (exception: when currently special regex characters are used).

*Example usage*

On master:
```
import salt.utils.event
event = salt.utils.event.MasterEvent('/var/run/salt/master')
print event.get_event(wait=100, tag='salt/job/.*/ret/<minion_id>', full=True)
```
On minion:
```
salt-call test.ping
```