PULSE test & fix riak_pipe_fitting

In an attempt to locate the source of `noproc` messages in issues 48 and 49, I wrote the attached tests to exercise part `riak_pipe_fitting` under PULSE. The specific part I was targeting was the 'eoi' behavior encountered when no workers are active. To facilitate Riak KV's mapreduce need to evaluate a reduce phase even if no inputs are received, `riak_pipe_fitting` will evaluate the worker behavior in-process in this case.

The PULSE test found that in addition to a `noproc` exit when sending synchronous messages to a sink that does not exists, `normal` (or any other reason) exits may happen if the sink exits *after* the synchronous message has been delivered. The `riak_pipe_sink:send_to_sink_fsm/5` function has been changed to catch these other exit reasons as well.

To see the tests fail, checkout the first (and second, if you wish) commits in this PR, and run `./rebar get-deps && make clean pulse` They should fail with a reason like (partial Eunit output):

```
    {function_clause,
        [{reduce_fitting_pulse,exit_reason_is_normalish,
             [{normal,
                  {pulse_gen_fsm,sync_send_event,
                      [<0.700.0>,{p">>...
```

And may also include:

```
Exit Reasons: [{client,after_eoi},
               {sink,normal},
               {fitting,
                   {normal,
                       {pulse_gen_fsm,sync_send_event,
                           [<0.675.0>,
                            {pipe_eoi,#Ref<0.0.0.11121>},
                            infinity]}}},
               {builder,normal}]
```

And:

```
::error:{assertion_failed,[{module,reduce_fitting_pulse},
                         {line,299},
                         {expression,"eqc : quickcheck ( eqc : numtests ( 5000 , prop_fitting_dies_normal ( ) ) )"},
                         {expected,true},
                         {value,false}]}
  in function reduce_fitting_pulse:'-death_test_/0-fun-2-'/0 [test/reduce_fitting_pulse.erl:299]
```

If they instead fail for a reason like `timeout`, you may be missing the [pulse_otp](https://github.com/Quviq/pulse_otp) package, or other PULSE components.

To see the tests pass, aadd third (and fourth, if you wish) commit to your checkout, and `make pulse` again.