Use default binaries

Instead of always having to specify the bin of an external downloader,
i.e `--curl_bin=curl`, … we add a default bin. We drop the '_bin'
suffix, allowing us to have the more user friendly `--curl`, … options.
One can still use a custom binary by giving it as the option value, ie.
`--curl=other_curl`.
Deprecate the wget_bin,... options

We keep the old options (for compatibility purposes), as there are
people using the script in cron jobs.

However, we exit the script with an error message informing the user to
change the option.
Merge pull request #123 from jonasdt/default_bin

Use default binaries when using external downloaders.