New Feature: Ability to retry operation on exception in DbManager

For certain exceptions (like deadlocks, timeouts, etc) it may be useful to retry the operation prior to returning from the DbManager.ExecuteOperation or DbManager.ExecuteOperation<t> methods.  Added one file (OperationExceptionRetryEventArgs.cs) and modified DbManager.cs to implement the feature.  Usage would be to handle the OnOperationExceptionRetry event and set the RetryOperation property of the EventArgs to true.  Any decision on when to retry for what exceptions and how many retry attempts can be implemented as desired by the handler of the event.

PS.  Sorry for not updating my fork correctly.  It shows me that this request is trying to include all changes since my fork was last updated.  I followed the GitHub instructions but failed to do it right.  Only the one new file and the minor changes to DbManager are needed for this.