Adds a murmur3 implementation for csum

This patch adds a murmur3 implementation (which speed-up the csum phase dramatically)
I also enabled -O3 switch from gcc (csum phase go down by 70% on my test file)
The code seems to work well on amd64 architecture, I cannot test it on i386 nor arm etc.

Signed-off-by: Jack Kenrick <github@jack.fr.eu.org>