Use IContentNegotiator to negotiate response based on request and configuration.
Send HTTP 404 Not Found when embedded resource is not found or ApiDeclaration is not found.
Merge branch 'content-negotiation' of https://github.com/chriseldredge/Swashbuckle into chriseldredge-content-negotiation

Conflicts:
	Swashbuckle.Core/Application/SwaggerSpecHandler.cs
	Swashbuckle.Core/Application/SwaggerUiHandler.cs
	Swashbuckle.Core/Swagger/ApiExplorerAdapter.cs
	Swashbuckle.Core/Swagger/ISwaggerProvider.cs
	Swashbuckle.Tests/HttpMessageHandlerTestsBase.cs
	Swashbuckle.Tests/SwaggerSpec/CoreTests.cs
Merge branch 'chriseldredge-content-negotiation'