Support parsing iso8601 dates

[emaste]

YYYY-MM-DD or YYYY-MM-DDThh:mm:ssTZD formats

>>> import parsedatetime as pdt
>>> cal = pdt.Calendar()
>>> cal.parse('1999-12-24')
(time.struct_time(tm_year=2013, tm_mon=4, tm_mday=4, tm_hour=14, tm_min=2, tm_sec=49, tm_wday=3, tm_yday=94, tm_isdst=1), 0)

[idpaterson]

YYYY-MM-DD is now supported in the latest commit in the master branch, but I was not able to figure out the best way to handle the time and time zone component. The date-only format is handled in a locale-agnostic way since this is meant to be an international date format.

[flight16]

Possibly a related bug? #133

[AgDude]

I would like this also. Based on the discussion in #97, parsedatetime is not timezone aware, so it can't fully support ISO 8601.

[ManojDatt]

Is there any way to specify the date format that parser should only understand, Like it should consider only DD/MM/YYYY but not MM/DD/YYYY ?