Allow drawing a horizontal or vertical gradient as the graph line


## Changes

* Added an enum `BEMLineGradientDirection` for determining whether to draw the line horizontally or vertically
* Added `gradientLine` and `gradientLineDirection` properties to `BEMSimpleLineGraph`
* Added `lineGradient` and `lineGradientDirection` properties to `BEMLine`
* Draw line with a gradient layer as the background

## Examples

[Horizontal Gradient](http://f.cl.ly/items/2p3S3a3T3f1v2H1B1438/iOS%20Simulator%20Screen%20Shot%20Nov%2013,%202014,%205.06.07%20PM.png) vs [Vertical Gradient](http://f.cl.ly/items/22083S2X0c123j290w32/iOS%20Simulator%20Screen%20Shot%20Nov%2013,%202014,%205.06.34%20PM.png)

```objc
size_t lineNumLocations = 4;
CGFloat lineLocations[4] = { 0.0, 0.5, 0.9, 1.0 };
CGFloat lineComponents[16] = {
    1.0, 1.0, 1.0, 1.0,
    1.0, 0.0, 1.0, 1.0,
    1.0, 0.0, 0.0, 1.0,
    1.0, 1.0, 1.0, 1.0
};
self.myGraph.gradientLine = CGGradientCreateWithColorComponents(colorspace, lineComponents, lineLocations, lineNumLocations);
self.myGraph.gradientLineDirection = BEMLineGradientDirectionHorizontal;
```


There were a few implementation changes to the line drawing, such as always using layers to draw the line. Thoughts?