Add support for reloading resources in Gradle plugin

Usage:

$ gradle bootRun
...

Edit files in src/main/resources and see the changes live in a web app
(e.g. static resources in /static).

The old functionality of bootRun has been replaced (since it didn't add
a lot of value and also didn't expose any JMV argument setters of anything).
This new feature set is backed by any existing "run" task configuration.

In addition autodetects a main class if there is one in the project
sources, so no need for mainClassName = '...' in build.gradle.

Applies the 'application' plugin (so no need to declare that either).

Fixes gh-225

spring-boot-samples/spring-boot-sample-simple/build.gradle

@@ -1,16 +1,16 @@
 buildscript {
+	ext {
+		springBootVersion = '0.5.0.BUILD-SNAPSHOT'
+	}
     repositories {
     	mavenLocal()
         maven { url "http://repo.springsource.org/libs-snapshot" }
     }
     dependencies {
-        classpath("org.springframework.boot:spring-boot-gradle-plugin:0.5.0.BUILD-SNAPSHOT")
+        classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
     }
 }
 
-ext {
-	springBootVersion = '0.5.0.BUILD-SNAPSHOT'
-}
 
 apply plugin: 'java'
 apply plugin: 'eclipse'