New Windows-style Git Logo

I have modified the icons and tested an installer build:

![screen](https://f.cloud.github.com/assets/1821299/140099/f60f0d40-7210-11e2-98c1-27f5675fc6cb.png)

For the installer sidebar image, I chose to enlarge the new Windows-style logo and exclude the "git" text, simply because I thought the yellow portion of the Windows style logo didn't contrast very well with the white background when set to a small size.  I was also bothered by the fact that the bottom tip of the diamond didn't line up with the dot over the "i" in "git," even though the diamond was centered correctly.  The "g" character is wide enough that it pushes the "i" and "t" over too much:
![bad_image](https://f.cloud.github.com/assets/1821299/138059/1cf7c85a-7187-11e2-9495-4c8375ef09c4.png)
Now that I see it here though, I kind of like it.  If you prefer that I use this image for the installer, just let me know and I can update it.

Note that this also updates the icon on the Git Bash, Git Gui, and a variety of other tools included in Git for Windows.

As I mentioned earlier, the logo lacks contrast at small sizes, so I decided to add a dark background to the icon to make it more visible:

![gbashdesktop](https://f.cloud.github.com/assets/1821299/138204/f463abb0-7191-11e2-80a5-f9df3a2fa6ae.png)

![taskbar](https://f.cloud.github.com/assets/1821299/138205/0afa2714-7192-11e2-8e19-cb126acfae92.png)

So there you have it!  Feel free to suggest any modifications!