Add C++11-style cbegin, cend, etc. to wxString's iterator interface

These c-prefixed functions always return a const_iterator.
The full list of functions added: cbegin(), cend(), crbegin(), crend().

Closes https://github.com/wxWidgets/wxWidgets/pull/314

docs/changes.txt

@@ -74,6 +74,7 @@ All:
 - Add support for loading dynamic lexer in wxStyledTextCtrl (New Pagodi).
 - Handle strings with embedded NULs in wxDataStream (Nitch).
 - Don't crash in wxTextFile::GetLastLine() if the file is empty (crohr).
+- Add wxString::cbegin() and cend() method (Lauri Nurmi).
 
 All (GUI):