add our own marshal format for Date

ruby 2.0.0p247
dump   8.563 µs/i ± 0.169 ( 2.0%) <=>    116785 ips
load   7.091 µs/i ± 0.446 ( 6.3%) <=>    141015 ips
reload 16.93 µs/i ± 0.346 ( 2.0%) <=>     59059 ips

jruby before with cached ajd
dump   5.991 µs/i ± 0.033 ( 0.6%) <=>    166926 ips
load   15.55 µs/i ± 1.890 (12.1%) <=>     64275 ips
reload 24.53 µs/i ± 0.461 ( 1.9%) <=>     40757 ips

jruby before without cached ajd
dump   8.306 µs/i ± 1.438 (17.3%) <=>    120389 ips
load   15.61 µs/i ± 0.384 ( 2.5%) <=>     64062 ips
reload 27.76 µs/i ± 1.315 ( 4.7%) <=>     36014 ips

jruby using [@dt, @of, @sg, @sub_millis]
dump   30.56 µs/i ± 0.059 ( 0.2%) <=>     32722 ips
load   123.4 µs/i ± 0.482 ( 0.4%) <=>      8099 ips
reload 165.6 µs/i ± 4.235 ( 2.6%) <=>      6038 ips

jruby using [@dt.getMillis, @of, @sg, @sub_millis]
dump   5.154 µs/i ± 0.044 ( 0.9%) <=>    194039 ips
load   10.85 µs/i ± 0.062 ( 0.6%) <=>     92154 ips
reload 19.41 µs/i ± 0.082 ( 0.4%) <=>     51505 ips