Added empty command, updated and reorganize docs

//cc @mwmiller @killerfish 

Implemented `duckpan empty` to clear the DuckPAN cache.

After adding the command to the README and man page, I kinda went a little crazy and reorganized things...Also added a few missing commands.

This also addresses #146
I forgot to add the new `cachesec` option to these files

```
    -c, --cachesec <number>
        Set the maximum number of seconds for which the asset cache is valid (default is 14400 [4 hours])

```

It's probably sufficient to have it here and not in cluttering the README.