Social Security Number generator

I've added two new methods, I've also added a social security number generator to `Factory::Personal`:

```ruby
Factory::Personal.social_security_number
#=> "123-45-6789"
```
To facilitate this, I've modified `Factory::Basic.number` to accept a :digits option to generate a number of a specific size, for example:

```ruby
Factory::Basic.number(:digits => 3)
#=> 213
```

`:digits` supersedes the `at_least/at_most` options accepted. It seemed to make sense to not try to support the combination of both.