Update RELEASE-NOTES.md
Update RELEASE-NOTES.md
Optimize ReferenceList::serialize
Merge pull request #261 from wmde/iterSerialize

Optimize ReferenceList::serialize
Update version number
Remove usage of Claims from StatementListDiffer
Improve naming in StatementListDifferTest
Merge pull request #262 from wmde/sld

Remove usage of Claims from StatementListDiffer
Merge pull request #263 from wmde/rename

Improve naming in StatementListDifferTest
Fix @since tag
Update links to original DataModel document
Don't use array_merge in getAllSnaks methods

It is unnecessary and more costly, performance-wise.
array_merge consumes the most memory of any method
used throughout the data model, Wikibase, and MediaWiki.

Avoiding array_merge here also helps improve the time it
takes to load an item in Wikibase.

In a medium-size item, with array_merge:

Entity::getAllSnaks - called 2 times, takes 109 ms
Claim::getAllSnaks - called 846 times, takes 188 ms
Statement::getAllSnaks - called 846 times, takes 148 ms

Without array_merge, on the same item:

Entity::getAllSnaks - 22 ms
Claim::getAllSnaks - 63 ms
Statement::getAllSnaks - 66 ms

StatementList::getAllSnaks appears unused
Merge pull request #266 from wmde/arraymerge

Don't use array_merge in getAllSnaks methods
Update RELEASE-NOTES.md
Update Item.php
Update Property.php
More cleanup in Claim::getAllSnaks