Improve date format performance

[TwP]

Since the Ruby 1.9 release, the Time#strftime method has supported native output of microsecond reoslution time stamps. Fixing the date formatting code in the Pattern layout to use this native %6N strftime format dramatically improves logging performance.

[TwP]

Here is the performance improvement of this change as evidenced by the test/benchmark.rb script:

== Warn (logged) == master ==
                 user     system      total        real
Logging:     3.140000   0.020000   3.160000 (  3.165859)
Logger:      2.110000   0.000000   2.110000 (  2.117281)
Log4r:       2.520000   0.010000   2.530000 (  2.538968)

== Warn (logged) == date-format-perf-upgrade ==
                 user     system      total        real
Logging:     1.860000   0.020000   1.880000 (  1.885256)
Logger:      2.160000   0.000000   2.160000 (  2.165824)
Log4r:       2.380000   0.010000   2.390000 (  2.391690)