add OnDatabaseScanStarted() to xbmc.Monitor()

@jimfcarroll I hope this is how you intended it to work :)

@Martijn please test if this is what you had in mind

```python

class MyMonitor( xbmc.Monitor ):
  def __init__( self, *args, **kwargs ):
    xbmc.Monitor.__init__( self )

  def onDatabaseScanStarted(self,database):
    print  "OnDatabaseScanStarted(%s)" % database 

```