Toggle raw types for signatures

They say an IRB session is worth a thousand words:

```
irb(main):008:0> commit = Rugged::Commit.lookup(repo, h.target)
=> #<Rugged::Commit:69881584773740 ...?
irb(main):009:0> commit.author
=> {:email=>"vicent@github.com", :name=>"Vicent Mart\303\255", :time_offset=>-28800, :time=>Sat Feb 23 12:39:11 +0100 2013}
irb(main):010:0> Rugged.raw_types = true
=> true
irb(main):011:0> commit.author
=> {:email=>"vicent@github.com", :name=>"Vicent Mart\303\255", :time_offset=>-28800, :time=>1361619551}
irb(main):012:0> 
```

/cc @rtomayko