Added function tabs()

* Added function tabs()
  * returns the specified number of tab characters (char(9))
Added function jd2iso8601()

* Added function jd2iso8601():
  * returns JD as date and time in ISO_8601 format
    * e.g. 2014-03-24T20:48:01+00:00
Added function int2str()

* Added function int2str():
  * converts an integer to a character string
Added function dbl2str()

* Added function dbl2str():
  * converts a double-precision real to a nice character string