Added my own theme, based on bira

Added a new theme based on bira and these async prompt
instructions, which adds the git status asynchronously, making it
significantly easier to work on huge repos (e.g. Chromium, WebKit).

themes/asyncbira.zsh-theme

@@ -0,0 +1,59 @@
+# ZSH Theme - Preview: http://gyazo.com/8becc8a7ed5ab54a0262a470555c3eed.png
+local return_code="%(?..%{$fg[red]%}%? ↵%{$reset_color%})"
+
+local user_host='%{$terminfo[bold]$fg[green]%}%n@%m%{$reset_color%}'
+local current_dir='%{$terminfo[bold]$fg[blue]%} %~%{$reset_color%}'
+local rvm_ruby=''
+if which rvm-prompt &> /dev/null; then
+  rvm_ruby='%{$fg[red]%}‹$(rvm-prompt i v g)›%{$reset_color%}'
+else
+  if which rbenv &> /dev/null; then
+    rvm_ruby='%{$fg[red]%}‹$(rbenv version | sed -e "s/ (set.*$//")›%{$reset_color%}'
+  fi
+fi
+
+setopt prompt_subst
+
+PROMPT="╭ ${user_host} %t ${current_dir} ${rvm_ruby}
+╰ %B$%b "
+RPROMPT=""
+RPS1="${return_code}"
+
+ZSH_THEME_GIT_PROMPT_PREFIX="%{$fg[yellow]%}‹"
+ZSH_THEME_GIT_PROMPT_SUFFIX="› %{$reset_color%}"
+ZSH_THEME_GIT_PROMPT_CLEAN="✔"
+ZSH_THEME_GIT_PROMPT_DIRTY="✗"
+
+ASYNC_PROC=0
+function precmd() {
+    function async() {
+        # save to temp file
+        printf "%s" "$(git_prompt_info)" > "${HOME}/.zsh_tmp_prompt"
+
+        # signal parent
+        kill -s USR1 $$
+    }
+
+    # do not clear RPROMPT, let it persist
+
+    # kill child if necessary
+    if [[ "${ASYNC_PROC}" != 0 ]]; then
+        kill -s HUP $ASYNC_PROC >/dev/null 2>&1 || :
+    fi
+
+    # start background computation
+    async &!
+    ASYNC_PROC=$!
+}
+
+function TRAPUSR1() {
+    # read from temp file
+    PROMPT="╭ ${user_host} %t ${current_dir} ${rvm_ruby} $(cat ${HOME}/.zsh_tmp_prompt)
+╰ %B$%b "
+
+    # reset proc number
+    ASYNC_PROC=0
+
+    # redisplay
+    zle && zle reset-prompt
+}