Add statemets as an object instead of a string in policy creation/update

[p16]

This PR changes the policy statemets property from a string to an object when creating or updating a policy.

Ref. issue #284

[paolochiodi]

I'd also modify all tests the send statements as strings (i.e. authorization, integration...)

I was also thinking, would be more clear it the expected object was something like

         statements: [
              {
                Effect: 'Deny',
                Action: ['documents:Read'],
                Resource: ['wonka:documents:/public/*']
              }
            ]

without the need of "Statement"?

I understand it would be more work for us for the mapping, but I'd thing it would make everything more clear

[p16]

@paolochiodi

modify all tests the send statements

will do, did not remember them :)

without the need of "Statement"

there's a ticket for that :) #197