Added/replaced some icons

According to my thread in the forum this pull request replaces/adds some icons. See here why: https://groups.google.com/forum/?fromgroups#!topic/chocolatey/dMBwD6Cm7m8

I replaced the old git logo with the official git icon. Hope you like that. ;-)

Note that GIF is a deprecated graphics format. It is not recommended because it has many disadvantages compared to PNG.

I added the SVG versions of icons too if they were available. Maybe one day we can use them, if folks stop using the crappy Internet Explorer versions < 9, which don’t support SVG.