Add select function to button

[scaddenp]

This simple addition added select and unselect method to buttons which add or remove a '.selected' class to the button. This is useful when controls activate a new mode (eg drawing) which captures mouse actions until deselected. A user can add a style like:
.leaflet-bar button.selected{
background-color: #666666 !important;
}

to style the button. I considered putting a default into the easybutton.css but though better of it.