Add support for axel downloader (--axel_bin)
Merge pull request #67 from FiloSottile/patch-1

Add support for axel downloader (--axel_bin)
Make the path of the `-n` option really optional.

This should fix #68.
Make passing optional path to `.netrc` also work for non-Windows OSes.
Fix warnings regarding `logging` from pylint.
Fix some long line and trailing whitespace warnings pointed out by pylint.
Restrict generality of a caught exception.

This way, we can know better what is the reason for a failure.
Avoid converting an exception to a string.

We already have the information at our disposal there as an integer, which
is exactly what we want to check.
Restrict generality of a caught exception.

I think that I found the most common errors that can happen while reading
credentials from a `.netrc` file, but the exception handling system will
surely let me know.