add timestamps extension

[simonkassel]

adds timestamps extension

closes #135

One question I have is about the apply method in TimestampsItemExt (and this question could be relevant to other extensions as well): if you pass null values for optional arguments defining properties that are already defined in that item, should the null value override the existing property?

So for example:

item already has published and expired defined.

if i do item.ext.timestamps.apply(unpublished=d), this will convert those other two properties to None. Should this be the case?

[lossyrob]

One question I have is about the apply method in TimestampsItemExt (and this question could be relevant to other extensions as well): if you pass null values for optional arguments defining properties that are already defined in that item, should the null value override the existing property?

That seems correct to me. I would expect that if None is passed in or if the param defaults to None, that would set the value accordingly

[lossyrob]

Looking good, comments inline.

[lossyrob]

Also note that you should add a CHANGELOG entry for this. Thanks!

[lossyrob]

Should have mentioned this before, but can you add an api.rst doc entry for the extension?

[simonkassel]

sure, will do