Use PCICt to take calendar into consideration

Hi @nperez,

This branch is for issue #81 (closed). I used as.PCICt to transfer the numerical values to dates, then changed them back to PCSIXct class. I created unit tests, including daily data using '360_day', '365_day', and 'standard' calendar, and one monthly data using 'standard' calendar.

Please let me know if you have any questions or suggestions, thanks!

Cheers,
An-Chi