Update the list of maintainers

pom.xml: 

 - Update the list of developers so that the proper list of maintainers (presently just imarinkov) can be picked up by the Jenkins wiki and displayed correctly.

 - Moved previous maintainers to the list of contributors

See https://maven.apache.org/pom.html#Developers

pom.xml

@@ -20,34 +20,31 @@
 
   <developers>
     <developer>
-      <id>npiguet</id>
-      <name>Nicolas Piguet</name>
+      <id>imarinkov</id>
+      <name>Ivaïlo Marinkov</name>
+      <email>[email protected]</email>
     </developer>
-    <developer>
-      <id>ohtake</id>
+  </developers>
+
+  <contributors>
+    <contributor>
+      <name>Nicolas Piguet</name>
+    </contributor>
+    <contributor>
       <name>OHTAKE Tomohiro</name>
-    </developer>
-    <developer>
-      <id>jieryn</id>
+    </contributor>
+    <contributor>
       <name>Jesse Farinacci</name>
       <email>[email protected]</email>
-    </developer>
-    <developer>
-      <id>ssogabe</id>
+    </contributor>
+    <contributor>
       <name>Seiji Sogabe</name>
       <email>[email protected]</email>
-    </developer>      
-    <developer>
-      <id>kbrandt</id>
+    </contributor>
+    <contributor>
       <name>Karsten Brandt</name>
-    </developer>
-    <developer>
-      <id>mixalturek</id>
-      <name>Michal Turek</name>
-      <email>[email protected]</email>
-    </developer>
-  </developers>
-
+    </contributor>
+  </contributors>
 
   <properties>
       <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>