Allow specification of default log location

Using `class << self` is fine, but I prefer the instance variables to stay outside as they were before. You can refactor to something like:

```ruby
@default_pid_path = './stripe-mock-server.pid'
@default_log_path = './stripe-mock-server.log'
class << self
  attr_accessor :default_pid_path, :default_log_path
  # ...
end
```