Date#initialize: do not compute @of and @sg, much faster to save them directly for now

ruby 2.0.0p247
Date.new 924.9 ns/i ± 13.60 ( 1.5%) <=>   1081082 ips

jruby: before
Date.new 20.72 µs/i ± 0.337 ( 1.6%) <=>     48260 ips

jruby: after
Date.new 10.61 µs/i ± 0.102 ( 1.0%) <=>     94207 ips

jruby: without creation of the DateTime
Date.new 7.483 µs/i ± 0.212 ( 2.8%) <=>    133643 ips

jruby: without computation of millis from ajd (through Rationals)
Date.new 4.980 µs/i ± 0.068 ( 1.4%) <=>    200788 ips

* ideal scenario is creating the DateTime from civil parameters directly (year, month, day)
  and never rely on @ajd, but it means rewriting many methods of Date