Update image processing in gulp

[mattstratton]

This needs to be slightly more robust.

For example, any image named logo-square needs to be resized to the proper widths, not percentages.

Same thing with all images in the sponsor directory.

This incorporates #153 and #134

[mattstratton]

I am 99% sure I have this solved now, but I'll let the build go and check it in the morning.

[mattstratton]

logo-square has been resolved, but sponsors are still somehow resizing themselves to 100px width.

[mattstratton]

Fixed in #270