* bindings.el (completion-ignored-extensions): Remove pdf and dvi

* bindings.el (completion-ignored-extensions): Remove pdf and dvi
extensions since they can be viewed with doc-view.

* files.el (auto-mode-alist): Make doc-view-mode the default mode
for pdf, ps and dvi files.

* doc-view.el: Make doc-view-mode the standard mode for viewing
pdf, [e]ps and dvi files and add binding C-c C-c to toggle between
text and image display.  Add binding C-c C-e to switch to an
editing mode.
(doc-view-ghostscript-options, doc-view-ps->pdf): Add "-dSAFER" to
avoid security problems when rendering files untrusted sources.