Fix mvn proxy

When doing a Debian `wheezy` build of `thrift` behind an HTTP proxy server, the Maven Ant Tasks fail because of network connectivity issues. This is apparently easily fixed by having a Maven `settings.xml` file containing suitable proxy info - except for one thing. I am building this inside a `cowbuilder` environment, so having a system-wide or user-local `settings.xml` isn't really a good idea for my use case.

It turns out after much research and Trying Of Other Things that the only way I could get this to work from within `cowbuilder` was to add rules to `lib/java/build.xml` to create a file, `maven-settings.xml`.

If the Ant `proxy.enabled` property exists, I add a `<proxy>` stanza to `maven-settings.xml` using the same settings as those used by the Ant `setProxy` rule, and add `settingsFile="maven-settings.xml"` to the `<artifact:dependencies>` rules. Otherwise I leave the `maven-settings.xml` file with an empty `<settings/>` stanza.

The `build.xml` already uses `proxy.enabled` and, in fact, the rule to create the `maven-settings.xml` file is in the same `<target name="proxy">` stanza as `setproxy`.

This works very well for me and I offer it as a pull request. I did test it on a non-proxy environment and it still works.

One other change I made: `dh_clean -k` is deprecated, and something it did made the build fail (a humongous `find` statement). I changed both of those in `debian/rules` to `dh_prep` as per the Debian recommendations. It would be great if you included this change, too.