OF-709: Replace servlet.jar (2.5) with servlet-api.jar (3.1)

On my Computer (MacOS, Java8) openfire doesn't start after the merge of b9a9655ac75e77add4e46e568e9bbe84df809c39.
Stacktrace:
```
2014.06.04 16:09:10 org.jivesoftware.openfire.container.PluginManager - Error loading plugin: /Users/sven/IdeaProjects/Openfire/target/openfire/plugins/admin
 java.lang.NoClassDefFoundError: javax/servlet/HttpConstraintElement
         at org.jivesoftware.openfire.container.AdminConsolePlugin.createWebAppContext(AdminConsolePlugin.java:337)
         at org.jivesoftware.openfire.container.AdminConsolePlugin.initializePlugin(AdminConsolePlugin.java:237)
         at org.jivesoftware.openfire.container.PluginManager.loadPlugin(PluginManager.java:481)
         at org.jivesoftware.openfire.container.PluginManager.access$300(PluginManager.java:80)
```

This pull request replace the servlet-api of mortbay jetty 6 with the servlet-api 3.1 of eclipse jetty 9.1.5. 
In addition I changed the filename to clarify the content of the jar. ( => It's just the api, not any rendering part for serlvets of jetty)