Add convenience functions for getting data type values from objects

targetted at Map reduce users, pass an object, get value. Avoids
us exposing the types.hrl to users. Also re-instate riak_kv_counter
but just for the `value` function, so as not to break old counter
mr jobs when we ship.
Fix function name typo
Fix typo in docs
Merge pull request #841 from basho/rdb/bug/counter-mr-backcompat

Backwards compat for users running map reduce jobs that use riak_kv_counter