Added png image and signature line options

Added options for outputting .png - not applicable to svg, base30, etc. I figured these are needed additions since a large signature div is helpful but outputting the .png at the same size is not. I added options to set the width and/or height, scale width/or height, proportion lock and edge trim. It can trim off the excess white space!!! Also, added new init option to leave off the signature since this appears in the .png. I just substituted the line with some HTML/CSS. I'm sure yall can figure that one out.

Observe the new options:

Scale picture by 1/2 and trim off white space:

	$('#signature').jSignature('getData', 'image', {
		'scaleX': .5,
		'trim': true
	});

Set the width to 200px and do not proportionally resize height:

	$('#signature').jSignature('getData', 'image', {
		'width': '200px',
		'lock': false
	});

Mixing the 'width' and 'height' options with 'scaleX' and 'scaleY' options will be confusing and I don't recommend it. Basically, the width and height gets established first before scaling. So specifying 'width' will result in the image being resized at that width but if you also indicate 'scaleX' then it will be scaled AFTER being resized. Just don't do it!!!

Options		Default		Description
-----------------------------------------------------------
width		'canvas.width'	Sets the png width
				(takes precedence before scaleX)
height		'canvas.height'	Sets the png height
				(takes precedence before scaleY)
scaleX		1		Sets the png width by multiplying
				current width. If 'width' is also
				set, that value gets scaled.
scaleY		1		Sets the png height by multiplying
				current height. If 'height' is also
				set, that value gets scaled.
trim		false		Trims off any clear edge pixel, ie
				if RGBA is 0,0,0,0, ie no color.
lock		true		Set the correct proportion for other
				dimension if not given. By setting
				a value for 'width', 'height' will
				set set proportionally. This also
				works for 'scaleX' and 'scaleY'

Here is how to remove the signature line:

	$('#signature').jSignature({
		'signatureLine': false
	});