Automatically detect 'development' profile

Detect when an application is running in development (by the presence
of a build file) and automatically add a 'development' profile.

Additional detectors can be developed by implementing the
`ProfileDetector` interface and registering with the `SpringApplication`

Fixes gh-296

spring-boot-samples/spring-boot-sample-simple/src/main/java/sample/simple/SampleSimpleApplication.java

@@ -20,6 +20,7 @@ import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.CommandLineRunner;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.ComponentScan;
 import org.springframework.context.annotation.Configuration;
 
@@ -43,6 +44,8 @@ public class SampleSimpleApplication implements CommandLineRunner {
 	}
 
 	public static void main(String[] args) throws Exception {
-		SpringApplication.run(SampleSimpleApplication.class, args);
+		ConfigurableApplicationContext context = SpringApplication.run(
+				SampleSimpleApplication.class, args);
+		System.out.println(context.getEnvironment().acceptsProfiles("development"));
 	}
 }