Alternative to setting beatStructure

[Trevor Daniels]

Some time ago, around Nov 2011, David folded $(set-time-signature ..)
into the \time command, see 
a512132fed73a94068b91fb0bab473319e477b6e.  

This change also provides an alternative to \set Timing.beatStructure, 
one which seems preferable to me.  The use of the new \time command 
is undocumented other than its use in a couple of snippets, for example see
Conducting signs measure grouping signs in
http://www.lilypond.org/doc/v2.17/Documentation/notation/beams#setting-automatic-beam-behavior

Should we document this use of \times in the main text, making changes like
this?

\relative c'' {
  \time 5/16
  c16^"default" c c c c |
  %\set Timing.beatStructure = #'(2 3)
  \time #'(2 3) 5/16
  c16^"(2+3)" c c c c |
  %\set Timing.beatStructure = #'(3 2)
  \time #'(3 2) 5/16
  c16^"(3+2)" c c c c |
}

Trevor