add index for properties ( userid, propertypath )

I enabled mysqls `log-slow-queries` with `log-queries-not-using-indexes` and used nautilus as well as the sync client to upload files to owncloud. The only query that seemed suspicious was the property resolving queries:
```sql
# Query_time: 0.003756  Lock_time: 0.000024 Rows_sent: 0  Rows_examined: 16197
SET timestamp=1366367355;
SELECT * FROM `oc_properties` WHERE `userid` = 'myuser' AND `propertypath` = '/clientsync/rahhhhhh.html';
```

there is also
```sql
# Query_time: 0.007433  Lock_time: 0.000020 Rows_sent: 0  Rows_examined: 22217
SET timestamp=1366367355;
SELECT `path`, `fileid` FROM `oc_filecache` WHERE `path` LIKE 'files/clientsync/rahhhhhh.html.part/%';
```

But I'll look into that in another PR. So, this is yet another small drop on performance for https://github.com/owncloud/mirall/issues/58 and https://github.com/owncloud/mirall/issues/331

@dragotin @danimo @bartv2 @icewind1991 @MTGap @DeepDiver1975