Edits to the HELIO section

Issues:
* The SunPy code doesn't work!  Here's what I encountered:
  * `helio.hec` has a dependency on the `requests` package, so I had to install it.  Is this requirement documented somewhere?
  * `hec.time_check_and_convert()` tries to pass a `%T` format code to `datetime.strftime()`, but this format code isn't supported, at least not formally.  I fixed it with `%H:%M:%S`
  * Then the example code starts choking on an empty VOTable.  That's when I decided to throw in the towel and give up.
* A reference is missing (DPS 2012)