Parses linear gradient definition
@param {String}
function parseLinearGradient(gradient) {
		var direction = defaultLinearDirections[0];
		
		// extract tokens
		/** @type StringStream */
		var stream = require('stringStream').create(require('utils').trim(gradient));
		var colorStops = [], ch;
		while (ch = stream.next()) {
			if (stream.peek() == ',') {
				colorStops.push(stream.current());
				stream.next();
				stream.eatSpace();
				stream.start = stream.pos;
			} else if (ch == '(') { // color definition, like 'rgb(0,0,0)'
				stream.skipTo(')');
			}
		}
		
		// add last token
		colorStops.push(stream.current());
		colorStops = _.compact(_.map(colorStops, normalizeSpace));
		
		if (!colorStops.length)
			return null;
		
		// let's see if the first color stop is actually a direction
		if (reDeg.test(colorStops[0]) || reKeyword.test(colorStops[0])) {
			direction = colorStops.shift();
		}
		
		return {
			type: 'linear',
			direction: direction,
			colorStops: _.map(colorStops, parseColorStop)
		};
	}