add search --creation-time/--last-change-time option

Hi,

I'm not familiar with git, not sure how to pull requests. I add search option --creation-time/--last-change-time option, which are very useful when search by date.

i pushed into my clone repo : https://github.com/chuckliu1979/pybugz
also i wrote a C version based on this pybugz, https://github.com/chuckliu1979/bugz, will make them sync up.
