Port manual to Documenter

Move all content to docstrings when they were missing or incomplete.
Fix many details in docstrings. Improve in particular docstrings
for sampling algorithms, analysis of signals, and stats models.

.travis.yml

@@ -16,3 +16,5 @@ git:
 #  - julia -e 'Pkg.clone(pwd()); Pkg.build("StatsBase"); Pkg.test("StatsBase"; coverage=true)'
 after_success:
   - julia -e 'cd(Pkg.dir("StatsBase")); Pkg.add("Coverage"); using Coverage; Coveralls.submit(Coveralls.process_folder())'
+  - julia -e 'Pkg.add("Documenter")'
+  - julia -e 'cd(Pkg.dir("StatsBase")); include(joinpath("docs", "make.jl"))'