[Form] Accept interfaces to be passed to "data_class"
[Form] Added test case for 4759e062ed004749dbdc2ba31aef0f8ac2601895
[PropertyAccess] Refactored PropertyAccessorTest
[PropertyAccess] Refactored PropertyAccessorCollectionTest
[PropertyAccess] Added isReadable() and isWritable()
[Form] ObjectChoiceList now compares choices by their value, if a value path is given
[PropertyAccess] Fixed CS and added missing documentation
[PropertyAccess] Removed the argument $value from isWritable()

To keep isWritable() and setValue() consistent, the exception thrown when only
an adder was present, but no remover (or vice versa), was removed.
[Validator] Refactored ValidatorTest and ValidationVisitorTest into an abstract validator test class
[Validator] Prototype of the traverser implementation
[Validator] Decoupled the new classes a bit
[Validator] Moved visitors to NodeVisitor namespace
[Validator] Added ObjectInitializer visitor
[Validator] Extracted code for group sequence resolving into GroupSequenceResolver
[Validator] Throw exception if ObjectInitializer is constructed without visitors
[Validator] Renamed $params to $parameters
[Validator] Made tests green (yay!)
[Validator] Added TODO reminder
[Validator] Added note about Callback constraint to CHANGELOG
[Validator] Completed GroupSequence implementation