Change citation style and text/scripts related to LD.

Changed citation style to MBE-like. Added more information about D' and
why it's horrible for flu sequences.
Also improved efficiency of LD calculator. LD results will be derived
from ~1600 complete influenza B genomes and will stick with statistics
related to r-squared. Might implement minor allele frequency cutoff
though.
Add Lewontin (1964), remove previous citation styles.