extconf.rb: remove "-Wl,--no-undefined" from ldflags

Remove ldflag "-Wl,--no-undefined" if existent to allow for loading
libyajl with dlopen.

This fixes building the native extensions on systems like gentoo which
set the ldflag "-Wl,--no-undefined" by default.
Merge pull request #11 from lebrinkma/fix-gentoo

extconf.rb: remove "-Wl,--no-undefined" from ldflags
add changelog for PR#11
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