README: Mention `.brew`

Ref. #83.
.osx: Make iTunes arrows go to your library by default (rather than the iTunes Store)
.osx: Disable the iTunes arrow links completely
Enable some Bash 4 features when possible:

- `autocd`
- Recursive globbing
Remove the feature tests for Bash 4 features and just fail silently

Hat tip: @svnpenn.
.bash_prompt: Tweak `$PS2`
.osx: Show hidden files by default in Finder

I’ve had the `show` and `hide` aliases for ages, but since I use `show` as
the default setting, it makes sense to include it in `.osx` as well.
.vimrc: Respect modeline by default
.vimrc: Enable per-directory `.vimrc` files

Ref. #86. Thanks to @necolas for the suggestion.
.osx: Increase sound quality for Bluetooth headphones/headsets

This has the same effect as setting Bluetooth Explorer → Utilities →
Special Options → Minimum bitpool to `40`. See
http://scott.dier.name/2009/10/osx-snow-leopard-a2dp.html.
.brew: Install PHP 5.4
.aliases: Support colorized output for `ls` 8.17 only (GNU coreutils)
.aliases: Detect which `ls` flavor is in use and use the appropriate flag for colorized output

Thanks to @divya for making me do this ;)
.aliases: Optimize the way we detect the `ls` flavor

Follow-up to 4169a34df1783b6b0727a527b867cefecaa069b5. Thanks to @janmoesen!
.aliases: Optimize the `ls` flavor detection even further

Ref. #89.
.osx: Hide the useless Time Machine and Volume icons

Fun fact — this is what the OS X Mountain Lion menu bar looks like
with all the menu extras (/System/Library/CoreServices/Menu Extras)
enabled: http://i.imgur.com/DHTuw.png
.osx: Comment out the line that enables the 2D Dock

I’ll try out the new frost glass Mountain Lion Dock for a while.
.osx: Add Twitter.app settings
.aliases: Make `update` update `npm` and its installed packages as well
.bash_profile: Source `~/.extra` before the other dotfiles

This is needed in case `$PATH` is expanded in `~/.extra`.