Added [-o,--overwrite] option

Hi,

I just started to use Droopy to transfer some information from an iPad app (actually running in the IOS simulator) to my Linux machine.  The app updates this data on a schedule, and I always just want the latest version.  However, by default Droopy creates -<n+1> files to avoid overwriting existing files, making it harder for my receiving program to get the latest data.

So this little patch adds a --overwrite option that disables this.