Change CSVBuilder to get column name from I18n

These changes will make the `CSVBuilder` try to get the column name through `Model.human_attribute_name`.
* If the attribute name is not defined on the locale, the name returned will no longer be "titleized", it will be "humanized".
* If the value specified to the `column` method is not a symbol, the builder will print it humanized.
* Very useful when using locales. :)

Instead of doing this:
```ruby
csv do
  column("Nome") { |r| r.name }
  column("Idade") { |r| r.age }
  column("Altura") { |r| r.height }
end
```

I'll do this:
```ruby
csv do
  column(:name)
  column(:age)
  column(:height)
end
```

And have my translations defined on the locale file:
```yaml
pt-BR:
  activerecord:
    attributes:
      person:
        name: Nome
        age: Idade
        height: Altura
```