Icon Fix re: issue #4

This is a potential fix for the issue at https://github.com/davkean/audio-switcher/issues/4 regarding the icon.

I have based this one on yours, and attempted to copy the design principles of the existing windows taskbar icons with high contrast and clear visibility regardless of the background. 

Here are two previews of the icon running in different colour schemes alongside the others:

![preview1](https://cloud.githubusercontent.com/assets/1923157/5562100/c62abfba-8e0c-11e4-825b-0067fe75bee6.PNG)
![preview2](https://cloud.githubusercontent.com/assets/1923157/5562101/c62fa390-8e0c-11e4-882b-d6f02044eaf3.PNG)

This is a subjective matter but I believe this solves the problem in a way that fits in with the existing surroundings.