Updating installation script

If we re-install PatchTester on a website, the copy of the folder failed because the destination folder already exists.
Using the option "force", the JFolder::copy won't generate an exception and copy the files even if the destination folder already exist.
Regarding the code of JFolder::copy, it can be useful to put this call in a try/catch ; otherwise the exception stop the all process.

Regards,
Jerome - Obsidev/HikaShop
Merge pull request #31 from obsidev/patch-1

Updating installation script