Refactor Gradle script naming and organization

- inline test-mvc and tiles3 .gradle files for clarity
 - rename merge-dist.gradle to merge-artifacts.gradle
 - polish Javadoc for merge-artifacts.gradle
 - move merge-artifacts.gradle into gradle subdirectory
 - move publish-maven.gradle into gradle subdirectory

spring-test-mvc/test-mvc.gradle

@@ -1,2 +0,0 @@
-ext.mergeIntoProject = project(':spring-test')
-apply from: "${rootProject.projectDir}/merge-dist.gradle"