1. Added readonly-attribute named view_type to Calendar class to set calendar view context i.e. weekly view, monthly view or yearly view

2. Added implementation to generate a monthly view calendar for given month and year combination. Added the following specs for the same under <<context "#show">>

   context "when invoked on .for_month.in_year" do
      it "returns a hash mapping week-day to an array of dates falling on the week-day in given month-year combination"

3. Refactored some code