Improve Entity::setId

* $item->setId( 42 ) is no longer deprecated (integer argument)
* $entity->setId is now deprecated (the whole method)
* $item->setId( $entityId ) is still deprecated though now is type hinted as such

Also brings us one step closer to killing Entity
Update Entity.php
Merge pull request #107 from wmde/setid

Improve Entity::setId
Don't rely on assert() to bail on assertion failure

The behavior of assert() is variable, depending on the value of the
assert.active / assert.warning / assert.bail / assert.quiet_eval runtime
options. To make the behavior of ClaimListDiffer::getDiffOp consistent and
indempotent, eschew assert() in favor of an explicit check.

Bugzilla bug: https://bugzilla.wikimedia.org/show_bug.cgi?id=65698
Merge pull request #120 from atdt/master

Don't rely on assert() to bail on assertion failure
Update README.md
Update README.md
Update .travis.yml
Update .travis.yml
Update README.md
Update README.md
Remoe @group WikibaseLib from PropertyNotFoundExceptionTest
Merge pull request #121 from wmde/PropertyNotFoundExceptionTestGroup

Remove @group WikibaseLib from PropertyNotFoundExceptionTest
Move diff classes into their own NS
Move diff code out of Entity and derivatives
Type hint against a more narrow interface
Add more fine grained tests for EntityDiffer
Add dedicated tests for ItemDiffer
Add getConstructionDiff and getDescriptionDiff to EntityDiffer
Minor style tweaks