The implementation of span_range (and also interval()) is surprising to me, and is not what I would expect from a range function.

            >>> start = datetime(2013, 5, 5, 12, 30)
            >>> end = datetime(2013, 5, 5, 17, 15)
            >>> for r in arrow.Arrow.span_range('hour', start, end):
            ...     print(r)
            ...
            (<Arrow [2013-05-05T12:00:0

This is related to #410 and #859, we currently only support a limited subset of possible values for RDATE, but we should support all the parameters in RDATE, the grammar for which is set out in RFC 5545:

       rdate      = "RDATE" rdtparam ":" rdtval *("," rdtval) CRLF

       rdtparam   = *(
                  ;
                  ; T

Is your feature request related to a problem? Please describe.
If I explicitly say Let's have a meeting at 3pm UTC and UTC is not my time zone, it would be helpful if I and other people in my time zone got the translation. That way I can be sure I chose the right time myself!

(Plus, this also means that when talking about events from UTC logs I can use TZB as a timezone translator! Yay!)

New locale handling is pretty cool, thanks. But I think I found a bug in relativeTime plugin.

So, there are three declensions of nouns in Russian: singular, dual and plural. The library does have h, hh and hhh for all of those (this is the case with other units as well), but it assumes that single-letter format is only used when the value of whatever unit is exactly 1. For example "ча