ZipStrings and Unicode

Commit 2c458be replaced the default code page with UTF-8, but ZipEntry.IsUnicodeText was still set to false by default. This makes it the default behaviour of SharpZipLib to create zip files that claim they are not coded using UTF-8 (but instead with what is presumed to be the locale default code page), but still containing UTF-8 encoded file names.

PR #255 rectifies that by setting the ZipEntry.IsUnicodeText default based on what ZipStrings.CodePage (previously called ZipConstants.DefaultCodePage) is set to.

It also moves the string transcoding from ZipConstants to ZipStrings as that class' name indicates another purpose. There are still wrappers for all public functionality that provides full backwards compability.