Track Logout of a customer

Added new column logged_out_at to add the datetime when a customer logs out.

The backend grid has two more columns: The logout date and the duration, a difference between login and logout. If logout is NULL then duration is also NULL.

The grid has an updated getRowUrl so that you can jump directly to a customer/edit form.

I'm stuck with the test VinaiKopp_LoginLog_Test_Model_ObserverTest::itShouldLogCustomerLogouts()

This one fails:

```
There was 1 failure:

1) VinaiKopp_LoginLog_Test_Model_ObserverTest::itShouldLogCustomerLogouts
Expectation failed for method name is equal to <string:load> when invoked 1 time(s).
Method was expected to be called 1 times, actually called 0 times.
```
Either at load() or at the session method: getVinaiKoppLoginLogId() which returns null instead of 815. After 1-2h searching I hope you will have answer where the bug is in this test :-)

0815 is just a random value.