makes olson_time_zones() more OS-agnostic

Should work for Ubuntu, as well as Windows. Other OS easy to account for, just to specify file location for `zone.tab`.

Tests included
adds warning to olson_time_zones

invokes warning if zone.tab is not found
adds warning to olson_time_zones

invokes warning if zone.tab is not found
Merge branch 'master' of https://github.com/ijlyttle/lubridate
tidies comment on olson_time_zone()
warning condition returns NULL in olson_time_zones

per Hadley's suggestion
changes warning to error; tweaks comments
fixes typo leftover from testing
Merge branch 'master' of git://github.com/ijlyttle/lubridate into ijlyttle-master