Improve handling of whitespace in %pythoncode

Previously SWIG looked at the indentation of the first line and removed
that many characters from each subsequent line, regardless of what those
characters were.  This was made worse because SWIG's preprocessor removes
any whitespace before a '#'.  Fixes github issue #379, reported by Joe
Orton.

Examples/test-suite/errors/swig_pythoncode_bad2.i

@@ -0,0 +1,13 @@
+%module xxx
+
+%pythoncode %{
+    def one():
+        print "in one"
+%}
+
+%pythoncode %{
+        print "still in one"
+
+    def two():
+        print "in two"
+%}