Add autoCloseDisabled attribute

The autoCloseDisabled attribute should be exported by paper-toast. If set, toasts do not automatically disappear after interacting with other elements on the page. Toasts would only disappear if they are closed manually or after the proper timeout.

The original and default behaviors of paper-toast are unchanged.
Add autoCloseDisabled attribute

Added comments for the new property added to paper-toast. The comments added mirror the comments from core-overlay.
Add autoCloseDisabled attribute (quotes)

double quotes added
Merge pull request #15 from afandria/master

Add autoCloseDisabled attribute