Python's enums are not extensible, values can't be added and they can't
be subclassed. This is problematic if weboob devel adds new values,
because stable weboob will not be able to use them. They would need to
be replaced by a default value or None, which loses information.

Instead, implement custom enums that contain plain constants and support
adding values aftewards.

Python 3.4 added new Enum types that enforce a value belonging to the
enumeration.
This commit retires our homemade enum and use Python's Enum type.
Python 2 compatibility is achieved with "enum34" package.

Housing and Calendar capabilities are updated to use those new enums.

Signed-off-by: Simon Lipp <laiquo@hwold.net>

[calendar] add timezone parameter to calendarevent in order order do get the good date/hours in ical exports

* define price as float
* add comments
* add new parameters (entry, max_entry, event_planner)
* add new method attends_event