Many calendar-source support

Make simple system to connect any calendar event source to calendar viewer.
For example, through simple pipe with standart input/output:
so we make /usr/bin/evolution-calendar-source-get-events written on c, /usr/bin/googlecal-calendar-source-get-events written on python and /usr/bin/jpilot-calendar-source-get-events written on another language.
#evolution-calendar-source-get-events 2010-20-04
(output:)
12:40 13:50 "Go to the market"
13:50 19:20 "Work"

#googlecal-calendar-source-get-events 2010-20-01
(output:)
12:40 13:50 "Google calendar event1"
13:50 19:20 "Google calendar event2"

It make possible for user to select any calendar source, also it make simply to write extension for developers
I can write googlecal and jpilot sources for example