Parsable standard formats for ZonedDateTimePattern

[GoogleCodeExporter]

Currently we have the "G" and "F" standard patterns for ZonedDateTime, but they 
don't allow parsing - because they use 
ZonedDateTime.GeneralFormatOnlyIsoPattern and 
ZonedDateTime.ExtendedFormatOnlyIsoPattern, which don't know which time zone 
provider to use.

However, they could easily be tweaked such that if a time zone provider *is* 
available, we use it. So then we could use:

    var pattern = ZonedDateTimePattern.CreateWithInvariantCulture("F", DateTimeZoneProviders.Bcl);

for example.

Original issue reported on code.google.com by jonathan.skeet on 12 Apr 2014 at 8:00