Make loading of ironoxide library easier for developers consuming the SDK

[ernieturner]

Instead of making ironoxide-java consumers manually call java.lang.System.loadLibrary manually with the name of the ironoxide library, we should wrap that internally and do it similar to how it works with loading NaCl. Their code automatically attempts to find the library in various paths and loads it for the caller.

private static final class SingletonHolder {
    public static final Sodium SODIUM_INSTANCE =
            LibraryLoader.create(Sodium.class)
                    .search("/usr/local/lib")
                    .search("/opt/local/lib")
                    .search("lib")
                    .load(LIBRARY_NAME);
}