Fix for Cyrillic text input on OSX

Would have affected other languages too; just most noticable for Cyrillic.
Merge pull request #405 from runrevfraser/refactor-syntax_unicode

Fix for Cyrillic text input on OSX
Merge branch 'refactor-syntax_unicode' into refactor-syntax_unicode_libicu
Add functions performing codepoint index mapping

These functions map between codepoint indices and UTF-16 code unit indices. The MCStringGetCodepointAtIndex function now behaves correctly in the presence of non-BMP characters.