Open modal without trigger of a button

This completes Dogfalo/materialize#187

I extracted the process of opening a new modal to a new function called `openModal()`
Now developers can do something like:
```javascript
$(document).ready(function() {
  $("#modalToOpen").openModal();
});
```
to open the modal within Javascript.

Binding to click event is still as is.

Updated the documentation as well.