add isoweek extractor to extract the ISO8601 week.
add isoweek accessor test for a tricky date
Couple of changes based on hadley's suggestions on my pull request
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
Merge pull request #208 from vitoshka/master

document the use of select_formats in parse_date_time
updating email address for maintainer
removing deprecated representation argument from s4 class definitions. Fixes #211
fixes typo leftover from testing
Merge branch 'master' of git://github.com/ijlyttle/lubridate into ijlyttle-master
ensure integer return value and handle dates in isoweek

Final division changed to integer division in isoweek.  In addition,
input coerced to POSIXct using ISOdate to ensure classes match for
substraction of nearest thursday (nth) and january 1st (jan1).
Previously, date input resulted in Date - POSIXct.
Add isoweek vector tests to test-accessors.R
Merge branch 'master' of git://github.com/jasonelaw/lubridate into jasonelaw-master
generating documentation for new functions
removing documentation for obsolete format_offset