More salient partially completed exercise icon (Fix for #2010)

Changed the partially completed icon on the knowledge map to something more salient.

Old:
![knowledge map old icon](https://cloud.githubusercontent.com/assets/6364266/2977326/23108ef0-dbaa-11e3-8cb3-e50bc4ec6faa.png)

New:
![knowledge map new icon](https://cloud.githubusercontent.com/assets/6364266/2977363/85ba8772-dbaa-11e3-88af-c022f87cc5dd.png)

I'm not sure what the changes are that are mentioned here, my only known change was editing the image found at ``` ka-lite/kalite/static/images/distributed/note-partial-26.png ```