Moved inplace add and multiply methods from UserString to MutableString.
Closes SF Bug #592573 where inplace add mutated a UserString.
Added unittests to verify the bug is cleared.
diff --git a/Lib/test/test_userstring.py b/Lib/test/test_userstring.py
index 5492f2e..67fbb5c 100755
--- a/Lib/test/test_userstring.py
+++ b/Lib/test/test_userstring.py
@@ -42,3 +42,4 @@
 
 string_tests.run_method_tests(test)
 string_tests.run_contains_tests(test)
+string_tests.run_inplace_tests(UserString)