Added reading passwords from file

You can now create a password list (default is ~/.unrar_passwords), which is a simple text file containing archive passwords (separated by newlines). If an archive needs a password, then unrarall will read the file in a top down manner and try to use each password until it finds the correct one or reaches EOF.

Please note, that this commit removes the ability to get asked for a password and input it manually. IMHO this is not needed, as this tool is clearly designed for mass extraction and should not wait for user input (this is also why the '-y' option is used in the code). If you need to extract an archive with a password, just put it in the passwords file  and remove the file afterwards (or just use plain unrar/7z).

This fixes arfoll/unrarall#13
Improved extraction of encrypted archives

Extraction of ecrypted (i.e. password protected) has been further improved. We now check if a rar-file is encrypted before we try to extract. This saves a lot of time and resources (especially when using large multi-part archives):

- When extraction of archives that are not password protected (but contain CRC errors) fails, unrarall will no longer try to extract them using every single password in the password list.

- When an archive really needs a password, unrarall will not try to extract it without a password first.
Use SIGTERM instead of SIGKILL

Named signals are much easier to read.
Fixed support for encrypted RAR file listings.

unrarall is now able to detect if the file listing is encrypted too. Prior to this commit, unrarall prompted for a password to get the file listing (and thus pausing batch archive extraction).