Improve interface for adding timestamps

Provide a simpler interface for adding <delay/> and <x/> timestamps to
stanzas.  This also makes sure that only one <delay/> tag and one <x/>
tag is added to a given stanza.
XEP-0198: Add timestamp to resent stanzas

When an unacknowledged stanza is resent from the Stream Management
queue, add a timestamp so that the receiving client can display the time
at which the stanza was originally sent.
Let CSI code add timestamp at later point in time

As a small optimization, make sure we won't add timestamps to presence
stanzas which end up being thrown away by the CSI code.
Add fractions of seconds to <delay/> timestamps

Include fractions of a second with XEP-0203 <delay/> timestamps, as
specified in XEP-0082.

	Old timestamp: 2014-05-19T11:55:00Z
	New timestamp: 2014-05-19T11:55:00.123Z
Let jlib use "B" instead of "w" to format integers

As a small optimization, use io:format's "B" control sequence to format
integers.  We don't need to let Erlang figure out the data type if we
already know it.
Merge pull request #377 from weiss/new-timestamps

Add timestamps to stanzas resent from stream management queue