Start adding UTF-8 support: first step

Add an internal function that guesses the encoding of a string:
ASCII, UTF-8, or CP437.
Add a test program and 3 test cases, one for each encoding.

--HG--
branch : HEAD
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index af09a39..4780b71 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -124,6 +124,7 @@
   zip_unchange_all.c
   zip_unchange_archive.c
   zip_unchange_data.c
+  zip_utf-8.c
 )
 
 INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR}