Support Travis for CI

This is a first shot at using Travis for CI. It also requires minor modifications to get `freenet-ext` build (see my fork). it does a complete build of Fred & `freenet-ext.jar`, and also makes the first build as simple as

~~~
git clone --recursive <repo url>
./build.sh
~~~

Although I have to admit I'd rather get rid of the `build.sh` script and polish the `build.xml` file somewhat to:

  * download & checksum-verify junit and bouncy-castle (if not already present?)
  * support the ant standard `test` target

This would reduce the `travis.yml` file to contain just the `language: java` line.

Another wart is that building contrib requires knowledge of Maven's `HOME` directory for reasons I don't fully comprehend. maybe the build script could check a handful default locations and error out if none of these exist?