Code cleanup in logging code

[Eilon]

Some misc code cleanup:

https://github.com/aspnet/Logging/blob/dev/src/Microsoft.Framework.Logging/DiagnosticsScope.cs

• Rename _disposedValue to _isDisposed

https://github.com/aspnet/Logging/blob/dev/src/Microsoft.Framework.Logging/Logger.cs

• Mark fields readonly as appropriate (including nested class's fields)
• Don't initialize fields to default values (e.g. bool to false)
• Fix names of fields to have leading underscore
• Rename disposedValue to _isDisposed
• Change ++i to i++ because reasons

https://github.com/aspnet/Logging/blob/dev/src/Microsoft.Framework.Logging/LoggerFactory.cs

• Add access modifiers to class members
• Trim extra newlines at the bottom of the file

https://github.com/aspnet/Logging/blob/dev/test/Microsoft.Framework.Logging.Test/DiagnosticsScopeTests.cs

• Rename 'DiagnosticsScopeTests' to 'DiagnosticsScopeTest'

[Eilon]

@sonjakhan