fix slow terminal init

Moved all completion.sh and .zsh files into the zsh directory, so I
don't end up scanning all of my dotfiles plus nvm each time a new
terminal is initialized.

zsh/git_completion.sh

@@ -0,0 +1,6 @@
+# this is specific to the location of the current version of git, installed by homebrew
+completion=/usr/local/Cellar/git/1.8.0.1/etc/bash_completion.d/git-completion.bash
+
+if test -f $completion; then
+    source $completion
+fi