add datetime encoding

necessary for ohai

json_gem spec complies with JSON gem format.  this also arguably
fixes issues with yajl-ruby where it does not encode date
objects correctly.
fix the timezone to UTC or Date specs will fail

only works in -800 right now
WIP
fix ruby 1.8.7 tests
add CHANGELOG.md
Merge pull request #9 from opscode/lcg/datetime-encoding

add datetime encoding