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 has this path set 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 a proper way to use JNI in
Play Framework. There is no documentation about it.