Handle existing files use-case for dev installer

Closes https://git.autonomic.zone/coop-cloud/abra/issues/98.

installer/installer

@@ -13,9 +13,12 @@ function install_abra_release {
 
 function install_abra_dev {
   mkdir -p "$HOME/.abra/"
+  if [[ ! -d "$HOME/.abra/src" ]]; then
     git clone "$GIT_URL" "$HOME/.abra/src"
+  fi
   mkdir -p "$HOME/.local/bin"
-  ln -s "$HOME/.abra/src/abra" "$HOME/.local/bin/abra"
+  ln -sf "$HOME/.abra/src/abra" "$HOME/.local/bin/abra"
+  echo "abra installed to $HOME/.local/bin/abra (development bleeding edge)"
 }
 
 function run_installation {