quickcheck test for riak_pipe_fitting

The riak_pipe_fitting_eqc:prop_eoi/0 property is an attempt to test riak_pipe_fitting's
behavior during 'eoi' propagation. The test sets up a fitting process, then starts zero
or more [fake] vnodes, then maybe sends eoi to the fitting, then starts zero or more
additional vnodes. All vnodes that receive a #fitting_details{} reply from the fitting
are expected to also receive 'eoi' messages from the fitting. The sink should also
receive an eoi.
broaden catch from exit:{noproc,_} to exit:_ in riak_pipe_fitting

This buglet was found by the new riak_pipe_fitting_eqc test. If the fitting was still up
when the gen_fsm message was called, but then stopped before replying, the exit reason
would be something like {normal, _} instead of the {noproc,_} already being caught.

This patch catches all exits to make sure execution ends up in the 'gone' case of
riak_pipe_vnode:new_worker, instead of in the catch clause. In the riak_pipe_fitting_eqc
test, the exit caused the test to abort, because it did not include the try/catch in its
fake vnode implementation.
Added eunit hook
Merge pull request #65 from basho/bwf-fitting-eqc

Quickchecking riak_pipe_fitting