.gitconfig: Explain `diff`-related aliases
.osx: Correct symlink for iOS Simulator

Ref. #135.
.osx: Restart Dashboard
.aliases: Add `dt`
.osx: Consistency tweaks
.aliases: Alias `sha1sum` if needed

Ref. #141.
.aliases: Add `dl`

Ref. #142.
.aliases: Make `update` update Ruby gems as well
.osx: Allow installing user scripts via GitHub or gists
.osx: Allow installing user scripts via Userscripts.org
.aliases: Make `update` run `brew cleanup`

Thanks to @patrickkettner for the suggestion on Twitter:
http://twitter.com/patrickkettner/statuses/254387082791903232
.osx: Add Mac App Store section

- Move existing command to new section
- Add debug menu command

Ref. #150.
README: Recommend sourcing the bootstrapper

Thanks to @svnpenn and @nvartolomei in https://github.com/mathiasbynens/dotfiles/commit/a77f4872f26d681a35cface9d6bcb4ad55fec752#commitcomment-2100022, and thanks to @janmoesen for the BASH_SOURCE pointer.
.osx: Add command to remove all Dock icons
.bash_prompt: Use `tput setaf 0` for white

Using `tput setaf 0` instead of `tput setaf 256` makes sure that `WHITE` is white instead of the previous color.

Thanks to @rudolph9 for reporting this in #153.
bin: Update to Bash 4.2.39
.gitconfig: Set `push.default = simple`

This will be the default in Git v2.
.bash_prompt: Fix `parse_git_dirty` for Git v1.8.0+

Git 1.8.0+ says “nothing to commit (working directory clean)”.
Older Git versions return “nothing to commit (working directory clean)”.

Thanks to @nicolahery in #143 for the suggestion.
.aliases: Add `map`

Ref. #157.
.functions: Make `gz` more awesome