dmbates/MixedModels.jl: Provide nAGQ=0 option for fitting GLMMs

Using fit!(glmm(...), nAGQ = 0) provides a faster optimization algorithm at the expense of some small loss of accuracy. For models with a large number of fixed-effects parameters relative to the number of covariance parameters there can be considerable speedup because nAGQ=0 profiles out the fixed-effects parameters as part of the PIRLS iterations, thereby reducing the dimension of the constrained, nonlinear optimization problem passed to an NLopt optimizer