org-publish failed on windows because of illegal timestamp filename

[Zhang Wei]

The org-publish-timestamp-filename function try to compose a timestamp
filename as something like "c:_BREP_org_notes.org.timestamp" which is
illegal on windows, as there's a colon `:' in the filename. The
following patch works for me:

*** org-publish.el.~1.22.~      2008-01-18 23:11:18.000000000 +0800
--- org-publish.el      2008-02-06 00:24:58.000000000 +0800
***************
*** 294,300 ****

  (defun org-publish-timestamp-filename (filename)
    "Return path to timestamp file for filename FILENAME."
!   (while (string-match "~\\|/" filename)
      (setq filename (replace-match "_" nil t filename)))
    (concat org-publish-timestamp-directory filename ".timestamp"))

--- 294,300 ----

  (defun org-publish-timestamp-filename (filename)
    "Return path to timestamp file for filename FILENAME."
!   (while (string-match "~\\|/\\|:/" filename)
      (setq filename (replace-match "_" nil t filename)))
    (concat org-publish-timestamp-directory filename ".timestamp"))