Improve 'build' folder ignores

Update .gitignore to only ignore 'build' folders in the project roots
rather than throughout the source tree.
Upgrade to xmlunit version 1.3
Fix [deprecation] compiler warnings

Fix deprecation compiler warnings by refactoring code or applying
@SuppressWarnings("deprecation") annotations. JUnit tests of
internally deprecated classes are now themselves marked as
@Deprecated.

Numerous EasyMock deprecation warnings will remain until the
migration to mockito can be completed.
Expose Gradle buildSrc for IDE support

Create 'spring-build-src' Gradle module that exposes the buildSrc
folder as an IDE project.
Develop a gradle plugin to add test dependencies

Develop a gradle plugin to automatically update testCompile dependencies
to include the test source sets of project dependencies.

Allows the gradle build to more closely mirror the way that tests run
inside eclipse.
Relocate MergePlugin package

Relocate the MergePlugin from org.springframework.build.gradle.merge
to org.springframework.build.gradle.
Move spring-build-junit into spring-core

Move code from spring-build-junit into spring-core/src/test along with
several other test utility classes. This commit removes the temporary
spring-build-junit project introduced in commit
b083bbdec720d3f5b1c7f065dfe8ccb7a7f3b201.