v0.6.12

Compared to v0.6.11:

• Take into account that BaseException may also raise ValueError if unable to decode JSON
• New get_or_none method for managers that returns None if not found, instead of raising an exception
• Allow collection endpoints to be defined without trailing slashes and make sure resource endpoints always have one
• expand option for resource-type fields, so that they will be represented as a dictionary with an id field instead of just the ID.