Bump StatsBase and StatsModels requirements

The release of StatsBase v0.19.2 resulted in this package failing to precompile because StatsBase and StatsModels both exported coefnames. StatsModels v0.2.0 resolved this problem. The main purpose of this release is to require these versions of StatsBase and StatsModels.

The dof and nobs methods have been updates and tests added.

Some of the documentation files have been updated by re-weaving the .jmd sources. Those that depend on the Gadfly package cannot be updated until a new version of Gadfly is released.