Additional workarounds for jupyter-kernelspec.exe

This proposed change amends and extends the previously included (so-called) hacks for Windows that were added for issue JuliaLang#363 (that are waiting on issue JuliaLang#448 for the jupyter project).  These changes allow for IJulia to work a version of Python that has been downloaded from python.org, where jupyter was pip installed with a version recent enough to include jupyter-kernelspec.exe and jupyter-notebook.exe, and where the end user has set the JUPYTER environment variable either at the system level or in ENV within their Julia session prior to building IJulia via Pkg.build.

If the issues that led to the discussion in JuliaLang#363 and jupyter issue JuliaLang#448 have been addressed, then the changes in this commit and PR can likely be ignored.

deps/build.jl

@@ -123,16 +123,26 @@ catch
             end
             python = abspath(Conda.PYTHONDIR, "python.exe")
         else
-            jk_path = readchomp(`where.exe $jupyter-kernelspec`)
-            jn_path = readchomp(`where.exe $jupyter-notebook`)
-            # jupyter-kernelspec should start with "#!/path/to/python":
-            python = strip(chomp(open(readline, jk_path, "r"))[3:end])
-            # strip quotes, if any
-            if python[1] == python[end] == '"'
-                python = python[2:end-1]
+            jupyter_dir = splitdir(jupyter)[1]
+            jks_exe = joinpath(jupyter_dir, "jupyter-kernelspec.exe")
+            if !isfile(jks_exe)
+                jk_path = readchomp(`where.exe $jupyter-kernelspec`)
+                jn_path = readchomp(`where.exe $jupyter-notebook`)
+                # jupyter-kernelspec should start with "#!/path/to/python":
+                python = strip(chomp(open(readline, jk_path, "r"))[3:end])
+                # strip quotes, if any
+                if python[1] == python[end] == '"'
+                    python = python[2:end-1]
+                end
+            else
+                jn_path = joinpath(jupyter_dir, "jupyter-notebook.exe")
             end
         end
-        run(`$python $jk_path install --replace --user $juliakspec`)
+        if isfile(jks_exe)
+            run(`$jks_exe install --replace --user $juliakspec`)
+        else
+            run(`$python $jk_path install --replace --user $juliakspec`)
+        end
         if endswith(jn_path, ".exe")
             push!(notebook, jn_path)
         else