Added assertion for updateLink

Added an assertion for string type. Since you could also pass objects
to `updateLink` (which would always result in false in the next if
statement) containing a link. This PR notifies creators of serializers
so they always use a string when handling links.

Added test for links with type other than string
Removed a duplicate line.
Added test case for push link with null value
Refactored InvalidError handling into a serializer concern.
Merge pull request #2392 from ahacking/serialization-for-invalid-error

Refactored InvalidError handling into a serializer concern.
Merge pull request #2354 from steffenbrem/update-links-assert-fix

Added assertion for updateLink
Fixing an issue when grouping requests could result in URL longer than configured length.
Merge pull request #2396 from grishace/uri_encoded_brackets

Fixing an issue when grouping requests could result in long URL
Nicer errors when pushing belongsTo/hasMany with invalid values
Merge pull request #2377 from wecc/nicer-push-errors

Nicer errors when pushing belongsTo/hasMany with invalid values
Fixes method_ajaxError link

Currently links to http://emberjs.com/api/data/classes/api/data/classes/DS.RESTAdapter.html#method_ajaxError 
Please review.
Merge pull request #2397 from vladikoff/patch-1

Fixes method_ajaxError link
disable canary until we figure out how to load Hbs

canary recently introduced Handlebars 2.0, and that's why canary is currently failing.
load handlebars 2.0 for canary
Merge pull request #2404 from emberjs/fix-travis-for-handlebars-2.0

load handlebars 2.0 for canary
Update model.js

Add documentation of the fact that reloading a model will fulfill with the reloaded model.
Merge pull request #2406 from kategengler/patch-1

Update model.js
Update explanation on how to run tests in browser
Merge pull request #2410 from BookingSync/update-contributing

Update explanation on how to run tests in browser
Add `createRecord` to PromiseArray so it proxies to ManyArray
Merge pull request #2411 from BookingSync/proxy-create-record-on-has-many

Add `createRecord` to PromiseArray so it proxies to ManyArray
createRecord on PromiseManyArray returns the new record instead of a Promise