Provide more methods/support for condidene intervals

[dwinter]

At present we only provide a parametric boostrap for generating bootstrap samples and a percentile boostrap to summarize them. This can be a problem, because the parametric boostraps are often biased (as a result of very rare alleles in some population samples/ and unsampled alleles in the "wider" pops).

We shuold:

• Include a resample-by-individual boostrap
• Include a normal-method (i.e +/- twice the standard error of the BS distribution) for estimating a CI
• Document these methods and their issues