Create a wrapper to GDL (gnu data language).

I am swapping IDl for GDL.
Synplot now runs with GDL.
Add an alternative way to run gdl. The other seems to be broken.

To run teh alternative one, the user has to install pidly.
Start notebook to test if I can call GDL from python.

GDL is the GNU Data Language. It is an open source project that
runs similar to IDL. My objective is to get rid of IDL for good.
Test running Synplot on GDL

It seems that the Synplot version that is installed with S4 do not
support GDL. But I could run a later version that I keep on a
private repository.

I should wirte this on the README. Also, on S4.synthesis.Synplot,
I should keep an option to run it also with IDL.
Synplot works with GDL, but with some execptions.

The version of synplot that is shipped with S4 do not work with
GDL.
Update README advertising user of GDL.
Create a generic wrapper to call shell commands
Add and option to run IDL or GDL.

Not yet tested due a unrelated bug on Synplot
IDL and GDL will not run quietly.

The prompt messagem that shows when GDL/IDL starts is now saved
in the log file. This will help the user identify if it is really
IDL or GDL that it is running.
Merge pull request #8 from gabraganca/GDL

Running Synplot with GDL