Change default Bitcask I/O mode to Erlang

Additional testing demonstrates that using Erlang's built-in I/O
provides more consistent behavior than the synchronous NIF approach.
While the NIF approach is known to provide higher throughput, the
non-deterministic impact on the Erlang VM from the use of blocking
NIFs outweighs the benefit. Defaulting to the safer option. Users
can manually revert to using NIFs if desired.
Merge pull request #78 from basho/jdb-erlang-default-1.6
Merge remote-tracking branch 'origin/1.6'

The 1.6 branch has diverged from master ... merging in recent mods.