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/zip.h b/lib/zip.h
index f8b55ac..5b3e2ce 100644
--- a/lib/zip.h
+++ b/lib/zip.h
@@ -162,6 +162,15 @@
 
 
 
+/* encoding type */
+enum zip_encoding_type {
+    ZIP_ENCODING_ASCII,   /* plain ASCII */
+    ZIP_ENCODING_UTF8,    /* possibly UTF-8 */
+    ZIP_ENCODING_CP437    /* Code Page 437 */
+};
+
+
+
 enum zip_source_cmd {
     ZIP_SOURCE_OPEN,	/* prepare for reading */
     ZIP_SOURCE_READ, 	/* read data */