Added mechanism to provide alternative cipher / hash implementations

All symmetric cipher algorithms and hash algorithms now include support
for a POLARSSL_XXX_ALT flag that prevents the definition of the
algorithm context structure and all 'core' functions.
diff --git a/ChangeLog b/ChangeLog
index 37308c1..5fd28f3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,11 +5,14 @@
    * Parsing of PKCS#8 encrypted private key files
    * PKCS#12 PBE and derivation functions
 
-Change
+Changes
    * HAVEGE random generator disabled by default
    * Internally split up x509parse_key() into a (PEM) handler function
      and specific DER parser functions for the PKCS#1 and unencrypted
      PKCS#8 private key formats
+   * Added mechanism to provide alternative implementations for all
+     symmetric cipher and hash algorithms (e.g. POLARSSL_AES_ALT in
+	 config.h)
 
 Bugfix
    * Secure renegotiation extension should only be sent in case client