Use rubygems and hpricot for to_xs in order to reduce dependencies

Hi,

As mentionned in the source code, the builder gem has strong dependencies. Running the script as is on Ruby 1.8 brings the following error : 

```
undefined method `encode' for Builder::XChar:Module (NoMethodError)
```

By using hpricot and its `fast_xs` method, it is possible to run the script on Ruby 1.8.

I also added a ```require 'rubygems'``` call that smoothes things and a ```Gemfile```.