Add support for CORS

[emxsys]

Add support for cross-origin resource sharing (CORS). By default, CORS will be disabled but easily enabled by uncommenting a section of the webapps/geoserver/WEB-INF/web.xml file

TODO:

• [ ] Download jetty-servlets-9.2.13.v20150730.jar from https://jcenter.bintray.com/org/eclipse/jetty/jetty-servlets/9.2.13.v20150730/.
• [ ] Add this file to the /webapps/geoserver/WEB-INF/lib folder in the distribution
• [ ] Add the following content to the /webapps/geoserver/WEB-INF/web.xml file

<!-- Uncomment this section to allow cross-origin resource sharing (CORS)
   <filter>
        <filter-name>cross-origin</filter-name>
        <filter-class>org.eclipse.jetty.servlets.CrossOriginFilter</filter-class>
        <init-param>
            <param-name>allowedOrigins</param-name>
            <param-value>*</param-value>
        </init-param>
   </filter>
    <filter-mapping>
        <filter-name>cross-origin</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
-->

• [ ] Add instructions for enabling CORS to the README