make Dispatcher configurable from nanocontainer.groovy

I made a patch to retrieve Dispatcher instance from application level PicoContainer (if not found, fall back to default ChainingDispatcher).

Here is a sample configuration for using FreeMarker as view:

[[nanocontainer.groovy]]
if (assemblyScope instanceof javax.servlet.ServletContext) {
pico = new org.picocontainer.defaults.DefaultPicoContainer(parent)
pico.registerComponentInstance(new org.nanocontainer.nanoweb.ChainingDispatcher(".ftl"))
return pico
} else if (assemblyScope instanceof javax.servlet.http.HttpSession) {
return new org.picocontainer.defaults.DefaultPicoContainer(parent)
} else if (assemblyScope instanceof javax.servlet.ServletRequest) {
return new org.picocontainer.defaults.DefaultPicoContainer(parent)
}

[[web.xml]]
<servlet>
<servlet-name>FreeMarker</servlet-name>
<servlet-class>freemarker.ext.servlet.FreemarkerServlet</servlet-class>
<init-param>
<param-name>TemplatePath</param-name>
<param-value>/</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>

<servlet-mapping>
<servlet-name>FreeMarker</servlet-name>
<url-pattern>*.ftl</url-pattern>
</servlet-mapping>