Add logs when loading rules

I think it can be useful to trace that we are loading the ndepend rules
when starting Sonar.

Change-Id: I083838de804d2c8b6830aaa8c6b54c421d5e5657
'Ndepend Rules URL' property must be optional

The plugin should work even if the property as these rules are optional.

Change-Id: I6fac7509c3e59d1f100dc9cf8eabb5488ddd7ed3