Add spring-build 2.2.3 and ensure build succeeds.

spring-build was previously included via an svn:external. Adding
directly to the source tree under Git to avoid the need for a git
submodule.

In order to build from any earlier commit, it is recommended to
export spring-build or symlink an existing copy into the root
of the spring-framework project and then build normally.

$ svn export https://src.springsource.org/svn/spring-build/tags/project-build-2.2.3 spring-build

spring-build/common/findbugs-exclude.xml

@@ -0,0 +1,6 @@
+<FindBugsFilter>
+     <!-- Exclusions -->
+     <Match>
+       	<Bug code="BAC,PS"/>
+     </Match>
+</FindBugsFilter>