Cache instantiated jQuery Form object when using multiple times

Just a slight performance increase here. Instead of using `$(this)` every time the form is accessed (a process that requires parsing the DOM each time), the instantiated jQuery object is stored in `$form` for frequent use. 

This is even how Stripe's [example form](https://gist.github.com/briancollins/6365455) is coded. 

Reviewing my PR, I just noticed that I subconsciously switched `$.attr` to `$.prop`. It works as is but I'm happy to revise this if it's a disagreeable change.