GGH-431 Fixed discovery of a start class

- Ensured that FunctionClassUtils performs additional check to ensure that located star class is SpringBootApplication
- Added additional lookup to look for Main-Class if nothing was found in Start-Class primarily to support Azure
- Updated Azure samples
- Updated documentation

Resolves #431

spring-cloud-function-samples/function-sample-azure/src/main/java/example/Config.java

@@ -22,7 +22,6 @@ import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
 import org.springframework.context.annotation.Bean;
 
-// @checkstyle:off
 @SpringBootApplication
 public class Config {
 
@@ -36,7 +35,6 @@ public class Config {
 	}
 
 }
-// @checkstyle:on
 
 class Foo {