Implement std::error::Error when not using failure

[faern]

I'm personally not a fan of using failure in libraries. Since it breaks the ecosystem into those who expose Fail errors and those who expose std::error::Error errors. Now when failure is optional here, it was very easy to add support for making the which error type a standard error.

[faern]

This makes it easier for libraries based on top of which that want to in turn expose an Error type implementing std::error::Error. Because now they (we) don't have to wrap this in failure::Compat<which::Error> in order to achieve that.

[harryfei]

Nice. Thanks.

[harryfei]

Released 3.1.0 in crates.io.