free/busy calendar support

Here is a first cut at free busy calendar support for your 0.9x dev branch

# Find when a user is busy
require 'time'
start_time = DateTime.parse("2013-02-19").iso8601
end_time = DateTime.parse("2013-02-20").iso8601
user_free_busy = cli.get_user_availability('joe.user@exchange.site.com', start_time, end_time, 'FreeBusy')
user_free_busy.calendar_event_array

# Find the user's working hours
user_free_busy.working_hours