doc: use submodule for JuliaDoc instead of package

The JuliaDoc package seems to be giving some people trouble due to
pip being unavailable, so pull it in as a submodule instead.

.gitmodules

@@ -7,3 +7,6 @@
 [submodule "deps/Rmath"]
  path = deps/Rmath
  url = git:https://github.com/JuliaLang/Rmath.git
+[submodule "doc/juliadoc"]
+ path = doc/juliadoc
+ url = git:https://github.com/JuliaLang/JuliaDoc.git

doc/Makefile

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line.
 SPHINXOPTS =
-SPHINXBUILD = sphinx-build
+SPHINXBUILD = PYTHONPATH=$(PYTHONPATH):juliadoc sphinx-build
 PAPER =
 BUILDDIR = _build
 JULIA = ../julia
@@ -45,8 +45,8 @@ help:
 clean:
  -rm -rf $(BUILDDIR)/*
 
-juliadoc-pkg:
- pip install --user --upgrade -r requirements.txt
+juliadoc-pkg: juliadoc/juliadoc/__init__.py
+ @-cd .. && git submodule update doc/juliadoc
 
 html: juliadoc-pkg
  $(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html