Fix issue #14.

Changes
======

Periods now use utc internally.

The constructor now handles timezones in the following fashion:

	1. Start and end are converted to utc for internal use. If tzinfo
	is specified and start or end is naive, it is going to interpreted
	as a time in the timezone specified by tzinfo.

	2. If a timezone is specified (tzinfo isn't set to None explicitly)
	Period.start and Period.end are in the timezone specified.
	
	3. If a timezone isn't specified naive datetimes are going to
	be returned by Period.start and Period.end

	4. Timezones now propagate through get_**** for example
	Months returned from Year.get_months() are now in the 
	same timezone.

Please review all files edited, and merge at your convenience.