Add "BOM" (Bill of Materials) for easier use with Maven

Guice extensions must generally have their version match the one
of Guice. Maven provides an easy way to make sure this is the case
with “BOMs” [1].

To make sure all your Guice dependencies (either direct or
transitive) use the same version (unless overridden), just add the
following to your <dependencyManagement> section:

    <dependency>
      <groupId>com.google.inject</groupId>
      <artifactId>guice-bom</artifactId>
      <version>${guice.version</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>

[1] https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#Importing_Dependencies
diff --git a/pom.xml b/pom.xml
index 7ded5d8..fb9040d 100644
--- a/pom.xml
+++ b/pom.xml
@@ -97,6 +97,7 @@
   </distributionManagement>
 
   <modules>
+    <module>bom</module>
     <module>core</module>
     <module>extensions</module>
     <!-- jdk8-tests module activated only when running under JDK8, below -->