Localization Support for Zoom Control

I added localization support for the zoom control. There was a pull request for this before: https://github.com/Leaflet/Leaflet/pull/1643 But the implementation was quite strange with language overriding. 

This implementation uses the control options to localize the strings. I also thought of another solution with: 

	options: {
		buttons: {
			zoomIn: {
				title: 'Zoom in',
				text: '+'
			},
			zoomOut: {
				title: 'Zoom out',
				text: '-'
			}
		}
	}

Let me know what you'd prefer and I will implement it.