Create namespace before requiring files.

Requiring files before defining the namespace was raising the `NameError (uninitialized constant Prawn::Svg)` exception. I tried running specs before doing this PR, but they're failing:

```text
github/prawn-svg/lib/prawn/svg/font.rb:40:in `block in load_external_fonts': undefined method `gsub' for nil:NilClass (NoMethodError)
```