Better ButtonWithLoading

Fix issue #110 - ButtonWithLoading is hiding MUI Button flexibility

Possible breaking changes: Please notice that I removed the default type="submit" from the button, meaning that you need to specify it, especially if you are working with forms.

New Global Snackbar

New implementation of global snackbar is simpler.

You will only need to add SnackbarContainer at the root of you app, then you can simply import the snackbarService and show a message as:

snackbarService.showSnackbar("message text", "message type");

No need for SnackbarContext and wrappers.

First release

The first version using new Material UI 4.0.