Performance improvements via simplifying and optimizing event emitter

This includes:

* Removed support for multiple eventnames from `on` and `off`.
* Removed defensive code around duplicate event registrations.
* Removing an event removes all handlers for that event name --
  there is no longer a way to remove a specific event handler.
* Optimized object shape for the event target mixin.

These changes are all in the service of performance. We used
a slightly modified version of cujojs/promise-perf-tests to
benchmark, which runs in node. We've attached them below.

@stefanpenner / @lukemelia

~
````
> promise-perf-tests@0.3.2 test /Users/lmelia/p/promise-perf-tests
> ./bin/run.sh

==========================================================
Test: promise-fulfill x 10000
----------------------------------------------------------
Name      Time ms   Avg ms   Diff %
when           26   0.0026        -
q              43   0.0043    65.38
rsvp PR        90   0.0090   246.15
rsvp master    94   0.0094   261.54

==========================================================
Test: promise-reject x 10000
----------------------------------------------------------
Name      Time ms   Avg ms   Diff %
q              38   0.0038        -
when           48   0.0048    26.32
rsvp PR        66   0.0066    73.68
rsvp master   100   0.0100   163.16

==========================================================
Test: promise-sequence x 10000
----------------------------------------------------------
Name      Time ms   Avg ms   Diff %
when           82   0.0082        -
q             191   0.0191   132.93
rsvp PR       240   0.0240   192.68
rsvp master   411   0.0411   401.22

==========================================================
Test: defer-create x 100000
----------------------------------------------------------
Name      Time ms   Avg ms   Diff %
rsvp PR        76   0.0008        -
when          208   0.0021   173.68
q             243   0.0024   219.74
rsvp master   276   0.0028   263.16

==========================================================
Test: defer-fulfill x 10000
----------------------------------------------------------
Name      Time ms   Avg ms   Diff %
when           85   0.0085        -
rsvp PR       120   0.0120    41.18
rsvp master   206   0.0206   142.35
q             247   0.0247   190.59

==========================================================
Test: defer-reject x 10000
----------------------------------------------------------
Name      Time ms   Avg ms   Diff %
when           80   0.0080        -
rsvp PR       131   0.0131    63.75
rsvp master   203   0.0203   153.75
q             228   0.0228   185.00

==========================================================
Test: defer-sequence x 10000
----------------------------------------------------------
Name      Time ms   Avg ms   Diff %
when          127   0.0127        -
rsvp PR       214   0.0214    68.50
q             291   0.0291   129.13
rsvp master   433   0.0433   240.94
````