646: toArray() fails to recognise Primary Key as DateTime

Task-Url: https://github.com/propelorm/Propel2/issues/issue/646

fixes #646

Added check for object in InstancePoolTrait to fix problem with DateTime
Object as PrimaryKey during call of save.

Changed build of $alreadyDumpedObjects in toArray method to allow DateTime
as PrimaryKey. Used hashCode() method for array key instead of getPrimaryKey() function.
Fix issues #676, #668, #694

Fix #676, #668 and #694 plus minor fixes on Propel commands.
update minimum requirements for symfony libraries

fixes #699 #698 #626
Propel2-619 Propel Query limit(0) not working

changed default value for limit to -1 in order to allow limit clause with 0.
Fixed some unit tests which failed because they expected a default value for limit with 0. Changed this to -1 because otherwise we can't use limit(0) clause.

fixes #619
Update database.dtd

Added missing reference of attribute `allowPkInsert` in the `table` element.
Also allow `behavior` to not have `parameter` children, like in timestampable behavior.
Made config file optional
Update database.dtd

added `vendor` child element for `index`
Made runtime.connections, runtime.defaultConnection, generator.connections and generator.defaultConnection optional
Merge pull request #700 from nymo/fix-699-698-626

update minimum requirements for symfony libraries to v2.4
Merge pull request #695 from cristianoc72/config-issues

Fix issue #676 and #694
Merge pull request #684 from nymo/fix-646-toArray-DateTime

Fix 646 to array date time
Merge pull request #701 from nymo/Propel2-619

Propel2-619 Propel Query limit(0) not working
Merge pull request #702 from sbarex/patch-1

Update database.dtd
Merge pull request #703 from mpscholten/optional-config-file

Optional config file and connection params. Fixed #680, #679.
Fix issue #697

Fix a problem in parsing xml configuration file.
Adjust Propel Configuration TreeBuilder to correctly parse associative arrays.
Adjust XmlToArray class to correctly convert numeric values.
This commit fixes #697