Implement check interval

[Pro]

Add function to automatically check each hour (time span can define user) if new files are available.

Because if running the script with crontab and it is already running because a download takes longer than the crontab interval files will be downloaded twice or even more often.

[arrizer]

I will implement a PID file to check if another instance of the script is already running.

[arrizer]

PID file single instance check is implemented since 0.6