Handle rank-deficient LinearModel. Also changes in docs (#207)

* Switch docs to Documenter

* Changes to allow CholeskyPivoted LinPred

* Coefficients for rankdeficient case

* Add allowrankdeficient to linear models.

* Add missing cell example for rank deficient

* Suggestions from @nalimilan

* Alignment

* Remove sphinx docs [ci skip]

* More tests and docs

Author: dmbates

.travis.yml

@@ -20,4 +20,6 @@ script:
   - julia --check-bounds=yes -e 'Pkg.clone(pwd()); Pkg.build("GLM"); Pkg.test("GLM", coverage=true)'
 
 after_success:
+  - julia -e 'Pkg.add("Documenter")'
+  - julia -e 'cd(Pkg.dir("Documenter")); include(joinpath("docs", "make.jl"))'
   - julia -e 'cd(Pkg.dir("GLM")); Pkg.add("Coverage"); using Coverage; Coveralls.submit(process_folder())'