Supported Hex string: 'x'

1. Added Hex string supporting for a little requirement from my job works.
I use a temp format name: `x`, because I found this format in [PHP](http://www.php.net/manual/en/function.pack.php) is `H` but this program have had `H` for unsigned short.
I also write a test script in `./test/hexstring.js`

2. Updated the dev dependencies versions
The package `should` doesn't work well in the previous version. So I updated the package info.

Hope to be useful.

Thanks.