Adding diff dataset command

[avdata99]

In progress
Related to #101

This PR creates the dpckan dataset diff and the dpckan resource diff command
This PR could be merged and these new functions could be useful.
They are not finished and they are in a very basic state.

Nice thing to add:

• Use hash functions to compare file resources
• Run a diff for each resource while diffing datasets
• Use this function to avoid updating a resource when is not necessary.
• Some fields require different comparison functions (e.g. resource formats are string and the case is not important). Maybe to define some case_insensitive_string_comp, tags_comp, org_comp or function like this could be re-usable and the code will be cleaner