giraph-debug caches instrumented jars

so it doesn't waste time on instrumenting the same jar again and again.

They are cached under `~/.giraph-debugger/jars/` with
`-instrumented.jar` suffix by default, and this location can be
customized with the environment variable `$JARCACHE_LOCAL`.
Merge pull request #50 from netj/cache-instrumented-jars

giraph-debug caches instrumented jars