Adding example for services defined in XML
use the new doctrine compiler pass to map the model classes directly
deprecate persistence specific classes
Update Canonicalizer.php

null !== ''

Checking for null is used in particular in the validation
Update Canonicalizer.php

Fixed return value
adjust to factory method
re-adding empty mapping files to keep BC compatibility
add support for mongo
do not have a fatal error if doctrine bridge is too old
adjust to https://github.com/symfony/symfony/pull/7755
add legacy symfony support
add support for couchdb
updated documentation
added upgrade instructions
put #1113 into this PR
How to configure Doctrine using yaml

I rewrote the solution proposed by mahok in http://stackoverflow.com/a/14988359/842697
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Removed unnecessary code
add deprecated errors to constructors, fix Upgrade.md
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