Allow setting column to NULL.

Special handling of NULL to make it possible to set columns to NULL.

This changes the behaviour of BO slightly, e.g:

* a text column set to null in database would remain null if set to '' in BO, now it will be set to an empty string.
* a int column set to null in BO would be set to 0, now it would be set to null.
Fix PHP segfault when missing function `Class::table_name`.

If the subclass is missing `Class::table_name` `__call` would be called, in turn calling `__get` which needs `Class::table_name` to work, repeat. Result is infinite recursion and a PHP segmentation fault.
Merge pull request #23 from ext/table_name_segfault

Fix PHP segfault when missing function `Class::table_name`.
Merge pull request #22 from ext/master

Allow setting column to NULL
Merge commit 'bc2527b'
Merge remote-tracking branch 'edruid/master'

Conflicts:
	BasicObject.php