Exports from StatsBase

[miromarszal]

I know it's easy to figure out from the source, but still, it would be nice to mention on the doc page that this is included:

 # StatsBase reexports
 dof, coef, nobs, mse, rss,
 stderror, weights, residuals

Especially because it's not a complete export of methods for regression models. It got me puzzled a bit at first, why things like coef(::LsqFitResult) would work and coeftable(::LsqFitResult) wouldn't. On on a related note, confidence_interval and estimate_covar could be linked with confint and vcov from StatsBase, like the other exports.