Privatized most of planner.rb for refactoring

* Made all constants private

* Made all methods except initialize, self.create, and generate_into
  private

* Tests all still pass, telling me that a) there are like NO unit
  tests up in this bidniss (like I didn't already know that) but more
  importantly b) as much as 92.5% of that class is SRP-violating
  code. ALSO something I already knew, but with it privatized all of
  that stuff is now a candidate for getting moved out to other classes
Renamed Week#date_label_for_week -> date_label
Updated notes
Changed use_thick/thin_pen to block methods

* They now get the old pen thickness, set the new pen thickness, yield
  the block, then restore the old pen thickness

* This did reveal some order-dependency drawing commands (the week
  date label was dependent on the pen having been left in the thick
  state from an earlier call) so the PDF magic checksum did change

* Renamed use_thick_pen -> with_thick_pen

* Renamed use_thin_pen -> with_thin_pen

* Factored out a common with_pen_width() method
Factored out common with_prawn_setting()
Added scoped_attr_accessor to remove warnings

* attr_reader inside a private block generates a warning

* Added scoped_attr_accessor gem to allow me to say
  "private_attr_reader :pdf, :start_date" and just have it work (and
  be warning-free)
Whitespace
Updated notes