Add support for retreiving EDF file

[joschaschmiedt]

In order to retreive the recored data via ethernet we need to implement the StartRecording and StopRecording methods with a filename:

[00 0 2 filename] start recording
[00 0 0 filename] stop recording (and copy .edf to the specified file)

Note: This cannot be developed without a real Eyelink system

[joschaschmiedt]

I implemented methods for stopping a recording in commit 59301bc. @KatharineShapcott , this needs to be tested though with a real EyeLink system.

[KatharineShapcott]

It is saving EDF files to the Eyelink computer with the correct names.