Everypolitician-popolo

* Fix the Person.twitter property when no Twitter details
were found.

* Fix __repr__ magic methods that broke on non-ASCII
strings.

* approx_dates is now a separate package on PyPi; this
version switches to using that.

* An exception is no longer raised if you call
Person.twitter when there are multiple Twitter usernames
found. The complete list can now be found with
twitter_all. (Similar changes have been made for phone,
fax and facebook.)
* ApproxDate now has a proper repr() representation
(i.e. the repr can be used to recreate the object)

* Fix a bug where properties returning dates didn't
consistently return ApproxDate objects.
* Make all Popolo objects hashable, so you can use set() to
remove duplicates, for example.
* ApproxDate can now express dates arbitrarily far in the
future or past.
* Add methods to Membership and Event to make it easy to
check if the object is current with respect to its
start_date and end_date.
* Make access to single related objects much more efficient.

* Add basic support for partial dates (e.g. '1999-12'). All
date properties now return ApproxDate objects.