Added 'npm test' script in package.json

There's a convention in the Node community of using `npm test` to test packages. That way, developers can easily test several packages without worrying about which particular test framework is used (e.g. nodeunit vs mocha vs qunit). I added a `test` script in `package.json` to enable the `npm test` command. I'm happy to report that all tests pass in Windows!

    $ npm test

    > smbhash@0.0.1 test c:\Documents and Settings\apenneba\Desktop\src\node-smbhash
    > nodeunit tests/


    common
    ✔ oddpar_success

    ntlm
    ✔ type1_success
    ✔ type2_success
    ✔ type3_success

    smbhash
    ✔ nthash_success
    ✔ lmhash_success

    OK: 21 assertions (30ms)