Change WeekRange calculation to use Date#cweek

Fixes #60 

Removes the custom week calculation in favor of Ruby Date's [calendar week](http://ruby-doc.org/stdlib-2.1.3/libdoc/date/rdoc/Date.html#method-i-cweek) implementation.  This should handle all time zones in a more robust manner.

> [Ruby Date's] calendar week is a seven day period within a calendar year, starting on a Monday and identified by its ordinal number within the year; the first calendar week of the year is the one that includes the first Thursday of that year.

Two changes are made to Date's calendar week to fit DoubleEntry's desired implementation:
* Days which are in a week ending in a new year are set to Week 1.
* Years that start on a Friday, Saturday, or Sunday need the `cweek` to be incremented by one.

For confirmation, here is a printout of each day of the year as well as it's corresponding week # and start date.  This matches up with the expected output.

https://gist.github.com/stavro/65f18bb6da2f5bf7fbe1


Since `Date` also responds to `to_date`, I would prefer just `date = time.to_date`. What do you think?