Set version to 1.1.0pre
Add convinience functions to add and remove classes on elements
Move set and unset of modal in own function
Reorder tests
Check if class is already set on element before applying again
Use correct path for modal.js in index file
Add new class names with space to element, reorder function calls in main handler
Use unset function to do necessary functionality
Rename function `_addEventListener` to `on`

This reflects implementation of libraries better and prevents
confusion in that _-previfixed variables are usually private
variables to an object.
This commit also changes the order of arguments for this function.
Rename function `_dispatchEvent` to `trigger`

This adds more consistency to the modal's API. `trigger` is used in
a lot of libraries to reflect displatching an event.
Apply all functions directly on the modal-object

This makes it more clean to use the modal's functions
Compile new CSS build while developing
Allow multiple, stackable modals

Modals are now stacked above each other if you call them from
another modal and the user has JavaScript activated. This allows
to guide the user through a process of several modals in a better
way.

There still need to be more tests and exampels written for that
specific case. Also browser tests need to be done.
The concept on mobile isn't yet fully developed and should be
redefined.

Closes #20.
Add function to keep focus in modal when tabbing through

When tabbing through the modal the focus remains within the modal
and re-starts at the beginning when it reaches the last tabbable
element within the modal.

Closes #26.
Add missing data-close label for mobile
Prevent close element to appear
Add styling and animations for stacked modals on small screens
Make .modal-close focusable again
Add forth AND back tabbing through the modal

More insights into this technique:
http://drublic.de/blog/accessible-dialogs-modals/

Reference #26.
Add more content in index.html to test scrolling behavior