Add native library directory before server starts.

Currently, only development mode(play run) runs well with sigar, because
sbt extracts native library files into `target/native_libraries/*bits',
and only development mode(play run) has this path in java.library.path.

This patch adds the native library directory into java.library.path
before server starts. But I'm not sure this is proper way to use JNI in
Play Framework. There is no documentation about it.