Added support for a partition key prefix based on a configuration file
value. If a value shall be loaded based on a configuration setting a
PartitionKeyPrefixKey can be used to load the value from the cloud
configuration file or the app/web.config appSettings.

+Some minor comment spelling edits.
Merge branch 'master' of https://github.com/wendelstam/NLog.Extensions.AzureTableStorage

Conflicts:
	NLog.Extensions.AzureTableStorage/AzureTableStorageTarget.cs