Pass JVM options to dx

This provides support for a `:dex-opts` keyword in the `:android` map. It works similar to leiningen's `:jvm-opts` keyword; you use it to pass JVM options to the dx tool. In my case, I need this because I have a big project so I need the dx tool to allocate additional memory. Similar to my previous pull request (I probably should have combined them).

>  :dex-opts ["-JXmx2048M"]