Allow gradle builds without version numbers

Add a gradle `ResolutionStrategy` that resolves missing version numbers
using the `spring-boot-dependencies` POM.

This commit also introduces a new `spring-boot-dependency-tools` module
that provides programmatic access to the `dependencyManagement` section
of the dependencies POM.

Fixed gh-262

spring-boot-tools/pom.xml

@@ -14,6 +14,7 @@
 		<main.basedir>${basedir}/..</main.basedir>
 	</properties>
 	<modules>
+		<module>spring-boot-dependency-tools</module>
 		<module>spring-boot-loader</module>
 		<module>spring-boot-loader-tools</module>
 		<module>spring-boot-maven-plugin</module>