Simplify interface

[superbobry]

I think we should choose between /2 and /3 functions and remove parse_* and format_* from the export list -- there should be only one way of doing things.

[si14]

I'm not sure that this is a good idea because:

1. we will break some code
2. sometimes the former is handier and sometimes the latter.

[superbobry]

1. This will be a separate release.
2. I don't see how tempo:format(..., {tag, Value}) is different from tempo:format_now(..., Value); plus, the former looks more extensible.

[si14]

Regarding 2nd point — well, sometimes it's just handier to avoid such boxing. But maybe you are right and we should drop it.