Pdpyras

* Address issue 37 and add other enhancements to ``iter_all``:
* Stop iteration in ``iter_all`` if the iteration limit (10000) is encountered, versus erroring out (because exceeding it will elicit a 400 response)
* Add the ability to set an initial offset via ``params`` versus always starting from ``offset=0`` in ``iter_all``
* Capitalize "constants"

* Define class variable ``retry`` initially as ``{}`` instead of ``None`` (`32 <https://github.com/PagerDuty/pdpyras/issues/32>`_)

* Added new idempotent resource creator function, :attr:`APISession.persist`
* Added the ability to use resource dictionaries (that have a ``self`` attribute) in place of URLs.

* Added support for using OAuth 2 access tokens to authenticate (`23 <https://github.com/PagerDuty/pdpyras/issues/23>`_)
* Added a property that indicates the access level/scope of a given API credential (`22 <https://github.com/PagerDuty/pdpyras/issues/22>`_)

* Fixed bug in :attr:`APISession.trunc_token`; property name typo causes ``AttributeError``

* The page size (``limit``) parameter can now be set on a per-call basis in any of the ``*_all`` methods (i.e. :attr:`PDSession.iter_all`) by passing the ``page_size`` keyword argument. If the argument is not present, the default page size will be used.
* The ``X-Request-Id`` header in responses is now captured in log messages to make it easier to identify API calls when communicating with PagerDuty Support
* Extended API call metadata is also now logged.
* The cooldown time between rate limit responses can optionally be randomized by setting :attr:`PDSession.stagger_cooldown` to a positive number.