Changing an async belongsTo association does not load unfetched record.

[Fixes #1567]
Merge pull request #2158 from Cyril-sf/cf/unload-belongs-to

Changing an async belongsTo association does not load unfetched record.
Update TRANSITION.md

Fixing syntax.
Merge pull request #2160 from gogogarrett/patch-1

Update TRANSITION.md
Fix typo
Merge pull request #2161 from mitchlloyd/typo-fix

Fix typo
Expose embedded options

Exposes embedded options helpers while keeping them private.

It is useful to have access to these, for example if one wanted to
make embedding records the default behaviour for their serializer they
could do that by overriding the helper methods.
Merge pull request #2163 from emberjs/exposeEmbeddedOptions

Expose embedded options
Batch hasMany requests with many id's to avoid max URL length issues