Import jsr305 and use it to mark @Nullable stuff. (#297)

pom.xml

@@ -58,6 +58,12 @@
 
   <dependencyManagement>
     <dependencies>
+      <dependency>
+        <groupId>com.google.code.findbugs</groupId>
+        <artifactId>jsr305</artifactId>
+        <version>3.0.2</version>
+        <scope>provided</scope>
+      </dependency>
       <dependency>
         <groupId>junit</groupId>
         <artifactId>junit</artifactId>