ensure we never create a content with a duplicated tag

[marcosh]

I realised it was possible to create a content with a duplicated tag (thanks to Quickstrom...).

To avoid this I decided to still use a List to represent tags but to use a smart constructor to remove duplicates using the nub function.

I chose not to use Set mainly because I would have lost the ability to have a Functor and Traversable instance for Content