Added buttonSize option to set general buttons size

[miguelmich]

Added support to change the default btn-sm button size class, in our design btn-sm is really small so I wanted to give it the option to change the global buttons size without affecting the current behavior:

$("#some-textarea").markdown({
  buttonSize: 'large'
});

It accepts there different values: normal, large, small and by default it keeps setting btn-sm, what do you think?

[miguelmich]

I also added the option to manage the buttons sizes by the btn-group-size option instead of adding the size class directly in each button:
http://getbootstrap.com/components/#btn-groups-toolbar