Added Alpha Channel increase file size

[dagjomar]

Comparing the file sizes of an original colored tile from OSM (http://tile.openstreetmap.org/3/2/4.png) @ 7.3 KB
and
The converted greyscale image, which is @ 14.4 KB (twice the size)

After looking at the properties of the two images, I found that the greyscale image has added Alpha Channel information, which results in the 7KB increase.

So... it could probably be optimized ^^

[Zverik]

Well, it's just twice, and I doubt that alpha channel is the sole cause for the size increase: new tiles differ from old, and compression algorithm may be different. If you have the time, pull request to fix this would be welcome :)