Feuerlabs stat combo

See [riak_core PR 487](https://github.com/basho/riak_core/pull/487):


The function identifying which system is used is riak_core_stat:stat_system(). This function is hard-coded to 'exometer', but can be changed to 'legacy' by setting the OS environment variable RIAK_CORE_STAT_SYSTEM to "legacy", and ensuring that the module riak_core_stat.erl is recompiled.