PRs from mabre merged

Fix string "None" being set as summary instead of object None
#58 by mabre was merged 8 minutes ago

Add created and last_modified event attributes
#57 by mabre was merged 10 minutes ago

Merge pull request #55 from infanf/master

v0.1.21

Merge pull request #55 from infanf/master

Merged pull requests from tdaff

Merge pull request #52 from tdaff/no-write-httplib
Disable httplib2 cache when no write permission

Merge pull request #49 from tdaff/rrule-until
Convert RRULE UNTIL with no tzinfo to UTC

v0.1.19

added support for default_span adjustment and additional event components #44 by tahouse

v0.1.18

Fixes wrong default value behaviour in Event.time_left

Merge pull request #42 from carloshanson/add-location

Merge pull request #42 from carloshanson/add-location

v0.1.16

Fix 'import UTC' error, caused by outdated requirement #37

v0.1.15

Add a 'recurring' attribute to Event. #35

v0.1.14

v0.1.14

v0.1.13

Fixed parsing when multiple rrules are present #32