Add time zone information

READY

1. Adds a `time_zone` identifier to all but 198 ZIP codes in the YAML file.

2. Updates the YAML file load path construction slightly to allow using the code without the gem installed (i.e. while developing)

3. Updates README with time zone info

I used information presented here http://stackoverflow.com/questions/2456324/mapping-us-zip-code-to-time-zone to match up time zones.  Specifically, the MySQL table here http://sourceforge.net/projects/zip2timezone/files/ and the Wikipedia list of time zones by state here http://en.wikipedia.org/wiki/List_of_U.S._states_by_time_zone

The ruby script I used to generate update YAML is here https://gist.github.com/Capncavedan/8263983