Add a test to the modal spec to ensure that the .mercury-modal and .mercury-modal-overlay elements get reused by all modals opened after the first one.
Reuse .mercury-modal and .mercury-modal-overlay divs instead of creating new ones every time a modal is opened.
Merge pull request #337 from nreckart/pr/reuse-modal-divs

Reuse .mercury-modal and .mercury-modal-overlay elements