A simple exponential backoff mechanism for PeriodicBatchingSink
Maximum 10-minute backoff interval rather than 15, just to be on the "safer" side.
.NET 4.0 build support for new batching back-off strategy.
Merge branch 'master' into feature-206-batching

Conflicts:
	src/Serilog.Sinks.AzureTableStorage/Sinks/AzureTableStorage/AzureBatchingTableStorageSink.cs
Merge branch 'feature-206-batching'