1.4.0: Merge pull request #14 from Antaris/develop

• Added support for {{#is...}} helper tags based on Dan Harper's Handlebars-Helpers library (https://github.com/danharper/Handlebars-Helpers)
• Added support for compiling against CoreCLR using net451 and dotnet5.4 TFMs
• Added support for resolving unknown expression values using the UnknownValueResolver delegate.
• Changed Func<object, string> instances to HandlebarTemplate to simplify the delegate types used for templates.
• Changed Func<RenderContext, string> instances to HandlebarPartialTemplate to simplify the delegate types used for partial templates.
• Changed Func<HelperOptions, string> instances to HandlebarHelper to simplify the delegate types used for helpers.