Improve Python docstring indentation handling

SWIG-3.0.5 and earlier sometimes truncated text provided in the docstring
feature.
SWIG-3.0.6 gave a 'Line indented less than expected' error instead of
truncating the docstring text.
Now the indentation for the 'docstring' feature is smarter and is
adjusted so that no truncation occurs.

Closes #475

Examples/test-suite/python_docstring.i

@@ -0,0 +1,98 @@
+%module python_docstring
+
+// Test indentation when using the docstring feature.
+// Checks tabs and spaces as input for indentation.
+
+%feature("docstring") docstring1 %{
+  line 1
+line 2
+
+  
+    
+line 3
+%}
+
+%feature("docstring") docstring2 %{
+line 1
+  line 2
+  
+      
+    
+  line 3
+  %}
+
+%feature("docstring") docstring3 %{
+    line 1
+      line 2
+
+          
+        
+      line 3
+      %}
+
+%feature("docstring") docstring4 %{
+	line 1
+	  line 2
+	
+	      
+	    
+	  line 3
+	  %}
+
+%feature("docstring") docstring5
+%{	line 1
+	  line 2
+	
+	      
+	    
+	  line 3
+	  %}
+
+%feature("docstring") docstring6
+{
+	line 1
+	  line 2
+	
+	      
+	    
+	  line 3
+}
+
+%feature("docstring") docstring7
+{
+line 1
+line 2
+line 3
+}
+
+%feature("docstring") docstringA
+%{    first line
+    second line%}
+
+%feature("docstring") docstringB
+%{		first line
+		second line%}
+
+%feature("docstring") docstringC
+%{  first line
+  second line%}
+
+%feature("docstring") docstringX "  one line docs"
+%feature("docstring") docstringY "one line docs"
+
+%inline %{
+struct DocStrings {
+  void docstring1() {}
+  void docstring2() {}
+  void docstring3() {}
+  void docstring4() {}
+  void docstring5() {}
+  void docstring6() {}
+  void docstring7() {}
+  void docstringA() {}
+  void docstringB() {}
+  void docstringC() {}
+  void docstringX() {}
+  void docstringY() {}
+};
+%}