Iso8601

Hi guys,

I have put some things together.

The biggest deal is the full and transparent support for ISO8601 offsets and Zulu time (i.e. Z UTC time). See the examples in ymd_hms and the parsing tests. strptime doesn't support +-08 and +-08:00 formats, so a slight overhead in speed for these two cases is unavoidable. it is still considerably faster than as.POSIXct.

I have rewritten some  docs and moved/removed/cleaned quite some code from last time. See individual commits.

Roxygen didn't work for me. So I did some manual work here and there. Please reroxegenize. 

Thanks,
    Vitalie.

fixes #150, #142, #141