Significant changes to MZDayPicker

- Got rid of MZDay and updated all other methods to expect NSDates as
arguments instead of MZDays. This has lots of obvious benefits ranging
from code simplification (don't need to constantly convert things back
and forth), eliminates unnecessary abstraction, and probably has some
minor performance improvements as well
- Created a new all-encompassing init method which has all the vital
components for the dayPicker. This solves a bunch of problems created
by the original MZDayPicker setting default start dates, end dates, and
current dates -- to be overridden later. Also, the init method now
expects a calendar, which helps to enforce that calendrical
calculations are made using the same point of reference
-Made some significant changes to the way that "day" units are computed
in the day picker. Rather than using imprecise (and redundant) second
division, we now use date components, which seems to have had a massive
performance improvement and forces the code to rely less on hardcoded
time intervals
- Got rid of a bunch of other unnecessary stuff (at least for Or Be
Square purposes) that wasn't helpful and in some cases caused bugs