Custom python interpreter for Windows platform

* ob-ipython.el (ob-ipython--launch-driver): `python-shell-interpreter`
  also considered on Windows platform.

Closes #96

Author: fleimgruber

ob-ipython.el

@@ -165,10 +165,12 @@ can be displayed.")
     (apply 'start-process name buf (car cmd) (cdr cmd))))
 
 (defun ob-ipython--get-python ()
-  (locate-file (if (eq system-type 'windows-nt)
-                   "python.exe"
-                 (or python-shell-interpreter "python"))
-               exec-path))
+  (let* ((python-candidate (if (eq system-type 'windows-nt)
+                               "python.exe"
+                             "python")))
+    (locate-file (or python-shell-interpreter
+                     python-candidate)
+                 exec-path)))
 
 (defun ob-ipython--create-kernel (name &optional kernel)
   (when (and (not (ignore-errors (process-live-p (get-process (format "kernel-%s" name)))))