Add support for md5 checksums

Color requests now support hex-encoded md5 checksums to be consistent with other avatar generation services like Gravatar. 

The implementation is pretty naive. Basically, the least significant three bytes of the 16-byte md5 digest are interpreted as the red, green and blue. As a result, only the last 6 hexadecimal digits affect the output color. On the other hand, this scheme distributes the space of md5 checksums evenly to the space of 3-byte RGB colors.