Change lookup by type to lookup by class

Currently, a prompt looks up the input element via it's type, e.g., "input[type=text]". This pull request adds the class "text" to the input and looks it up based on that class. 

This is beneficial for two reasons:
1. In cases where the input type has been changed (for example, I am changing it to a password type in one instance), bootbox will still find the correct element.
2. The look up will be faster as the browsers native document.getElementsByClassName can be used.

(I did it because of a specific case of #1, but #2 is still beneficial.)