Added nullable attribute and validation

[Mallone63]

'Nullable' could be useful when 'required' is set to 'no'.

[AirisX]

Hi @gonditeniz ,
have you any plans on merging that?

[Grokzen]

@mall The code you have submited is lacking some basic things. One is tests for the new attribute/feature. Next is example code for this feature. Third is general documentation changes for this. Fourth is updates in changelog file. Without fixing these basic things i will not merge this PR.

[Grokzen]

@Mallone63 This MR needs some more work in order to be mergable. If possible please rebase your changes ontop of the latest master branch in my repo inside your fork, fix the comments that you have here above and finally submit a new MR and i will get this feature in to the code base. I will not do more work on this feature inside this MR scope until that is done. I like the feature and want it in but you have to complete it first.

[Mallone63]

@Grokzen PR has been rebased on current master branch. Added tests, example \ docs, and changelog update.
#117