autodetect nosetests2 or nosetests in run-tests

'nosetests2' will be used if available, 'nosetests' if not.
On systems like arch linux 'nosetests' defaults to the python3 version
causing tests to fail. Thus nosetests2 has preference.

fixes #9

run-tests

@@ -15,7 +15,11 @@ exitonfailure () {
 }
 
 # run actual tests
-nosetests
+if command -v nosetests2; then
+    nosetests2
+else
+    nosetests
+fi
 exitonfailure $?
 
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