Add tgit.icon project config

This is a new feature in TortoiseGit 1.8.6,
which allows to show ILSpy icon as taskbar overlay icon

![tgit-ilspy-icon](https://cloud.githubusercontent.com/assets/791115/2624444/729c3d56-bd3d-11e3-8d37-f066d62736d9.png)

![tgit icon-help](https://f.cloud.github.com/assets/791115/1322125/1325c0b6-3414-11e3-8331-b2259e6ee61c.png)