find xmlstarlet even if it is called xml

which is the case with at least the Homebrew (OS X) and pkgsrc (many)
package managers.

FIXME: doesn't check if 'xml' is xmlstarlet
TODO: fail explicitly if no

fetch-tweets.sh

@@ -10,6 +10,11 @@ then
 	exit 1
 fi
 
+# xmlstarlet is often just xml
+if ! which xmlstarlet >> /dev/null && which xml >> /dev/null; then
+        alias xmlstarlet=xml
+fi
+
 twitter_total=$(curl -s "http://api.twitter.com/1/users/lookup.xml?screen_name=$1" |
 xmlstarlet sel -t -m "//users/user/statuses_count" -v .)