Review ILogger log method

[glennc]

Need to have severity, object, callback to stringify.

Add bool IsEnabled(severity)

[Eilon]

Specifically, in this interface: https://github.com/aspnet/Logging/blob/dev/src/Microsoft.Framework.Logging.Interfaces/ILogger.cs

WriteCore needs to be focused on just writing log messages. It should not need to return a bool to indicate success or enabled status. We should add a new IsEnabled method (not sure if it needs the severity param...) to check specifically whether the log is enabled.

[Eilon]

@sonjakhan

[davidfowl]

While we're at it, should we call WriteCore Write/Log?

[sonjakhan]

was pushed this morning, coherence-signed has passed :)