Adding created_at, updated_at and sheet_type properties to view items

* Adding model for PersonalAccessToken authentication information

* Making the signin request accept generic credentials

* Adding login sample

* adding sign in tests for tokens

* Fixing lint issues

* moving auth credentials to a property

* Making PAT use a specific sign in method, to avoid misuing the feature attempting to use it against older versions of the server

Add support for getting and setting permissions for all content types. 
This is accomplished with a few patterns, some new, some old.
1 - We now have a `_PermissionsEndpoint` much like tags, that gets composed into a permissions sub-endpoint for each type. Calls to the type endpoint pass through to the sub-endpoint, just like tags. Item models treat permissions like any populated property.
2 - Permissions are returned as a list of rules, where a rule is a grantee-capability pairing.
3 - We have another sub-endpoint for DefaultPermissions, for Projects and (in another PR), databases.
4 - We have a new `as_reference` and and `to_reference` method on `User` and `Group` types, that let's us create an id-only object for use in permissions parsing, and as a short-hand for those objects when you don't need all the other properties. I think this could expand to all item types, but I haven't yet hit a strong need for it so skipped that for this already giant PR.

Looks Good!