New function for retrieving Olson-style time zone names

Olson-style names are the most readable way of specifying time zones, but the only way to find out what values you can use is to dig around inside the R install directory (or hunt on Wikipedia).

I've added the function `olson_time_zones` that parses the file that ships with R, and returns the names in alphabetical order, or from West to East.  Usage is

    olson_time_zones()
    #[1] "Africa/Abidjan"     "Africa/Accra"       "Africa/Addis_Ababa"
    #[4] "Africa/Algiers"     "Africa/Asmara"      "Africa/Bamako"     
    #etc

    olson_time_zones("longitude")
    #[1] "Pacific/Midway"    "America/Adak"      "Pacific/Chatham"  
    #[4] "Pacific/Wallis"    "Pacific/Tongatapu" "Pacific/Enderbury"
    #etc