SHA2 renamed to SHA256, SHA4 renamed to SHA512 and functions accordingly

The SHA4 name was not clear with regards to the new SHA-3 standard. So
SHA2 and SHA4 have been renamed to better represent what they are:
SHA256 and SHA512 modules.
diff --git a/ChangeLog b/ChangeLog
index 5a44205..66a5c9c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -21,6 +21,7 @@
      and maximum protocol version
    * Renamed error_strerror() to the less conflicting polarssl_strerror()
      (Ability to keep old as well with POLARSSL_ERROR_STRERROR_BC)
+   * SHA2 renamed to SHA256, SHA4 renamed to SHA512 and functions accordingly
 
 Bugfix
    * Fixed parse error in ssl_parse_certificate_request()