remove logging, add config-settings for caching, add docs

docs/settings.txt

@@ -0,0 +1,21 @@
+settings in tytond.conf
+
+[cashbook]
+# enable caching of values, stores values between
+# browser requests, speeds up the retrieval of data.
+# default: yes
+memcache = yes
+
+# Enables synchronization of the cache between multiple
+# server instances.
+# default: yes
+# Requires the setting :
+# [cache]
+# clean_timeout=0
+sync = yes
+
+# Enables caching of the cache key for currency rates.
+# Reduces the time required to determine the cache
+# key for currency values.
+# default: yes
+cache_currency = yes