Add a type-appropriate equality operator

Hi Dao Bing,

I'm interested in using your Lunar Date module in a web application, and when testing it out noticed that it doesn't override the equality test yet (so while ld == ld work, LunarDate.today() == LunarDate.today() does not since Python is simply checking if it's comparing the same objects).

Attached is an added __eq__ operator that compares the time delta, in the style of the existing __lt__ and __le__ operators. Cheers!