Add .DS_Store to .gitignore
Add file browser entry in webjump site

`webjump' uses completing-read to get site names, but it returns nil if
user does not input any character. Then browse-url will open the
directory of the current buffer in the system file browser, which might
be useful in some cases. I explicitly add a dummy entry to make this
special case clearer.
Enable auto-complete in ielm
Enable smartparens except for lisp family modes
Enable ac-clang only if clang is installed
Move ido-ubiquitous and flx-ido into ido settings

The two packages enhance ido.
Bind `ido-switch-buffer' to F2
Add package `keyfreq'

It might be useful to record frequently used keys, and give them shorter
keybindings.
Add package `ggtags'

This mode provides an interface to use GNU global.
Allow magit restore window config after quitting
Increase maximum number of items saved by recentf
Fix indentation error in python settings
Fix more warnings found by flycheck

Set parameters of clang and emacs-lisp checkers.
Refine the navigation functions

* Omit "M-g", and make "M-." and "M-," behave as the same as possible
  when editing Emacs lisp, Python, and C/C++.
* Refine keybindings in ggtags.
* Enable ace-jump-mode mark ring sync.
* "C-M-b" and "C-M-b" now navigate local mark ring.