Gem structure cleanup and UTF-8 encoding bugfix

Here is a fix about UTF8 encoding (wrong size inserted in chars instead of bytes).
I also cleaned a bit the structure and updated the gemspec to new convetions.

---

BTW In case you didn't know, you can now use bundle's standard rake task to deal with gems:

```
$ ruby-bencode (master) > rake -T
rake build         # Build bencode-0.8.1.gem into the pkg directory
rake install       # Build and install bencode-0.8.1.gem into system gems
rake release       # Create tag v0.8.1 and build and push bencode-0.8.1.gem to Rubygems
```
For example, once you merged this, you can do `rake release` to tag the commit, build the gem and push it to rubygems automatically.