Deprecate the keccak function

It just added support for text/hex to eth_hash's version, but that has a
significant performance impact and was not really used anywhere, so
deprecating it now so that it can be removed in a future version.

Closes: ethereum#95

eth_utils/crypto.py

@@ -1,3 +1,5 @@
+import warnings
+
 from eth_hash.auto import keccak as keccak_256
 
 from .conversions import (
@@ -6,4 +8,7 @@
 
 
 def keccak(primitive=None, hexstr=None, text=None):
+    warnings.warn(DeprecationWarning(
+        "The `eth_utils.keccak()` function has been deprecated and will be removed in a "
+        "future version. You should update your code to use eth_hash.auto.keccak instead."))
     return keccak_256(to_bytes(primitive, hexstr, text))