Support TZif format v3

On at least openSUSE 13.1, some of the files in `/usr/share/zoneinfo` already use the TZif v3 format, making the unit tests for `std.datetime` fail.

The only difference between v2 and v3 is an extension of the POSIX-TZ-style
string, which Phobos doesn't use anyway, so the fix is trivial.

For reference:
ftp://ftp.iana.org/tz/code/tzfile.5.txt