Added `mergedProperties` to ember-metal, Ember.Route's `events`

/cc @lukemelia, @kselden

Added `mergedProperties`, the object-merging sibling of
`concatenatedProperties`. Properties specified in the
`mergedProperties` array will not overwrite but rather
merge into such properties defined in extended parent
classes or mixins. Merged methods can call _super

Also added it to the `events` hash in `Ember.Route`,
where it is most useful. Will add it to
Controllers per #2884 in a separate PR once
this is merged.
Fixed a typo in documentation for {{log}}
Merge pull request #3090 from graup/patch-1

Fixed a typo in documentation for {{log}}
Added the ability to send a context with actions in components
Merge pull request #3092 from sebastianseilund/component-action-context

Added the ability to send a context with actions in components
Fixed some docs for Ember.Route
Merge pull request #2888 from machty/merged-properties

Added `mergedProperties` to ember-metal, Ember.Route's `events`
Remove unused property

`Ember.CoreObject#isInstance` is referenced from only test code.
Merge pull request #3096 from tricknotes/remove-is-instance

Remove unused property
Update CONTRIBUTING.md

Rephrase "building ember" to only suggest the Vagrant route if the user is having issues installing some of the build tool dependencies.
Controller action handlers are now expected to be defined on an `events` object, instead of directly on the controller.

This makes controller action-handling consistent with Route
action-handling.

The existing behavior continues to work but is deprecated.