Update CHANGELOG

Update CHANGELOG

Version 0.10.0 CHANGELOG

Update CHANGELOG for version 0.9.0

Add upgrade guide

Make implementation compatible with PHP 5.3

Merge pull request markuspoerschke#31 from php-workx/dates_with_utc_v…

…alue

Dates with utc value

Update README.md

Merge pull request markuspoerschke#28 from navarr/several-attendees

Add Several Attendees

Merge pull request markuspoerschke#27 from php-workx/extended_timezon…

…e_support

Extended timezone support