Use zsh globbing instead of ls + grep in xc function

Uses zsh arrays and globbing to find .xcworkspace and .xcodeproj files.

plugins/xcode/xcode.plugin.zsh

 #xc function courtesy of http://gist.github.com/subdigital/5420709
 function xc {
-  xcode_proj=`ls | grep "\.xc" | sort -r | head -1`
-  if [[ `echo -n $xcode_proj | wc -m` == 0 ]]
-  then
+  local xcode_proj
+  xcode_proj=(*.{xcworkspace,xcodeproj}(N))
+  if [[ ${#xcode_proj} -eq 0 ]]; then
     echo "No xcworkspace/xcodeproj file found in the current directory."
   else
-    echo "Found $xcode_proj" 
-    open "$xcode_proj" 
+    echo "Found ${xcode_proj[1]}"
+    open "${xcode_proj[1]}"
   fi
 }