Change application:get_env/2 -> app_helper:get_env
Update tools.mk to 0.5.6
Fix Edoc syntax errors in riak_client.erl
Fix Edoc errors in riak_kv_test_util

Because we don't compile docs with TEST defined, the @doc comments
remain, but the function definitions themselves aren't seen by edoc.
This makes edoc think there are just bare `%% @doc ...` comments. This
change just removes the `@doc` bit.
Fix EDoc syntax errors in riak_index.erl
Fix Edoc syntax errors in riak_kv_bucket
Fix Edoc syntax errors in riak_kv_counter

This resulted in a cryptic function clause error from the edoc code
itself.
Fix Edoc syntax errors in riak_kv_crdt
Fix Edoc syntax errors in riak_kv_delete

Removes completely wrong arity spec. Useless.
Fix Edoc syntax errors in riak_kv_gcounter
Fix Edoc syntax errors in riak_kv_memory_backend

Since we don't compile docs with TEST defined, this just ends up being a
bare `@private` tag, which EDoc doesn't understand.
Fix Edoc syntax errors in riak_kv_multi_backend

* Move module-level doc to top-level
* Remove tags from functions only defined with TEST
Fix Edoc syntax errors

s/@Doc/@doc
Fix Edoc syntax errors in riak_kv_gcounter

The .erl extension causes Edoc to fail with a function clause error.
Fix Edoc syntax errors in riak_kv_stat_bc
Fix Edoc syntax errors in riak_object
Fix Edoc syntax errors in riak_kv_yessir_backend

Some other changes clean up the XML in the module doc.
Fix Edoc syntax errors

s/@Doc/@doc
Fix Edoc syntax errors in riak_kv_wm_counter
Fix Edoc syntax errors in riak_kv_wm_crdt