RELEASE_1_4_19 => v1.4.19 commit
RELEASE_1_4_20 => v1.4.20 commit
Implement Countable for Doctrine_Null

Currently, count(new Doctrine_Null()) returns 1. This means
Doctrine_Hydrator_RecordDriver::setLastElement() will try to call
getLast() on instances of Doctrine_Null and fail.

Compare this to count(null), which gives 0: if Doctrine_Null is to
represent null it should give the same count.

This commit was originally
dominics/doctrine@754ca890faf76f87b5a3fe6707ba497099dfe314.
Fix for DC-797: one-to-one or none causing dirty hydration

This patch modifies the record's _isValueModified method to mark a
change from null to Doctrine_Null (or vice versa) as not a modification.

This bug causes any record containing an empty one-to-one relations
(nullable foreign key) to be hydrated as dirty off-the-bat, when no
modifications have taken place. This causes knock-on problems with
things like the Timestampable behavior.

This bug is discussed at doctrine/doctrine1#8
Replaced README with changes from mainstream version