add ability to use gant cache from the gant ant task

I was looking to speed up the load time when running gant tasks from ant and found that gant already supported caching the compiled `.gant` file results, but that ability was not exposed to the `<gant>` task. This change adds that support by adding two new attributes to the `<gant>` task: useCache and cacheDir

Here is an example from my project utilizing these new attributes:

    <gant
        file="ant/build.gant"
        target="${target}"
        inheritAll="true"
        useCache="true"
        cacheDir="build/temp/gant"
      />