Upgrade Maven Javadoc plugin to version 2.9.1

** Bug
    * [MJAVADOC-354] - isValidJavadocLink is too strict: doesn't
support unnamed package
    * [MJAVADOC-358] - maven-javadoc-plugin is missing threadsafe on
some goals like test-jar

** Improvement
    * [MJAVADOC-361] - Improve linguistic style of bottom
    * [MJAVADOC-363] - [Patch] Port maven-javadoc-plugin from
httpclient 3 to httpclient 4
    * [MJAVADOC-370] - Javadoc vulnerability (CVE-2013-1571 [1], VU#225657 [2])

** Task
    * [MJAVADOC-364] - Upgrade QDox to 1.12.1

Bug 
* [MJAVADOC-349] additionalJOption only supports one additional option 
* [MJAVADOC-342] An incomplete fix for the NPE bugs in AbstractJavadocMojo.java 
* [MJAVADOC-312] having parenthesis as part of a path causes ArrayIndexOutOfBounds exception 

Improvement 
* [MJAVADOC-350] Allow include/exclude at the class level 
* [MJAVADOC-348] Update "javaApiLinks" links 
* [MJAVADOC-343] maven-javadoc-plugin is not marked as @threadsafe, which leads to a warning shown during parallel builds 
* [MJAVADOC-334] Patch to make <additionalDependencies> easier and fix typo on field name 

Task 
* [MJAVADOC-346] use maven-plugin-tools' java 5 annotations 
* [MJAVADOC-296] explain resources directory