fix 'SaveAs' in case of overwriting existing project

Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/BeremizIDE.py", line 959, in OnSaveProjectAsMenu
self.CTR.SaveProjectAs()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 600, in SaveProjectAs
self.SaveProject(old_project_path)
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 579, in SaveProject
self._getProjectFilesPath(self.ProjectPath))
File "/usr/lib/python2.7/shutil.py", line 200, in copytree
os.makedirs(dst)
File "/usr/lib/python2.7/os.py", line 157, in makedirs
mkdir(name, mode)
OSError: [Errno 17] File exists: '/tmp/tests/project_files'

Project overview
================

In order to target the widest possible range of programmable devices and keep efficient, Beremiz use C code as an intermediate language. 

To be executed, C needs to be compiled. `GCC <http://gcc.gnu.org>`_ serve that purpose perfectly.

PLC program is expressed in languages defined in IEC-61131, including graphical languages. Thanks to PLCopen TC2, those graphical languages have a standardised representation, in XML.

To be continued.