Collections renaming fix

When renaming a database, we were stuck with this error:
```
BSONCommandError['command renameCollection failed because the 'ok' field is missing or equals 0'] with original doc {
  ok: BSONDouble(0.0),
  errmsg: BSONString(access denied; use admin db)
}
```

In fact, the `renameCollection` command must be run against the `admin` database, therefore it needs to be run as admin, specifying renamed collection and target full namespaces.

ReactiveMongo now reflects this behavior.

See http://docs.mongodb.org/manual/reference/command/renameCollection/ for more information about this command.