Add ability to fill the top and/or bottom of a graph with a gradient

## Changes

* Add `gradientTop` and `gradientBottom` properties to `BEMSimpleLineGraphView` class
* Add `topGradient` and `bottomGradient` properties to `BEMLine` class
* Rename `gradient` property of `BEMLine` to `lineGradient` to differentiate from other gradient properties
* Fill top and/or bottom gradient when the corresponding property is set
* Added an example usage to the sample project, which should make a graph appearing something like this:
![Example gradient fill](http://f.cl.ly/items/0F0U2q083K3p2t461v2b/iOS%20Simulator%20Screen%20Shot%20Nov%2010,%202014,%202.25.08%20PM.png)