Fix for 3259: No messages support in example plugin

Changes by file:

. resource/admin/example-plugin/ExamplePlugin.java
  - support for messages
  - fix for PluginResult.Startup refactoring

. resource/admin/example-plugin/example_plugin.properties
  - 3 messages used in the example plugin (english/default)

. resource/admin/example-plugin/example_plugin_fr.properties
  - 3 messages used in the example plugin (french)

. resource/admin/example-plugin/build.xml
  - automatic generation of messages from properties files located in
    resource/messages while keeping package name.
    (eg. resource/messages/my/example/example_plugin.properties will
    generate a message class with package name "my.example")

. resource/Messages.java.stub
  - forced underlying properties files to be located in /messages within
    jars file (eg MY_JAR_FILE:/messages/example_plugin.properties)
  - support for messages available from extensions' classloaders

. src/messages/src/org/opends/messages/Message.java
  - typo in javadoc

. build.xml
  - added message template used by generator in the build-tools.jar
  - added the "resource/messages" directory at the root of the example
    plugin
  - added example properties files in the example plugin zipfile

. src/build-tools/org/opends/build/tools/GenerateMessageFile.java
  - stopped assuming messages are targeted in package name
    org.opends.messages
  - template for message class generation now retrieved from the
    build-tools.jar when generating messages for extensions
  - for user messages, set destination files automatically based on the
    source full filename and apply basic pattern
    (eg. source=resource/messages/my/example/example_plugin.properties
    sets destination to
    src-generated/my/example/ExamplePluginMessage.java)