Ensure there are spaces after headers and tables

* Headers beginning with #s should have blank lines both
  before and after; add this spacing.
* Add blank lines around tables (for the same reason);
  this closes #189.
* Remove spurious blank line at the ends of curl examples.
* I noticed there are sometimes two blank lines *before* a
  heading; this looks like it may be down to the way the
  templates are coded (I'm a ruby newbie). I think it is
  not a showstopper (and this patch does not affect it).