Fix `StandardErrorStreamLogTest`.

It tested the `StandardOutputStreamLog` instead of the
`StandardErrorStreamLog`.
Extend the readme.

The readme has only shown the status of the CI server, but there are
more important things that should be part of the readme.
Add support for EditorConfig.

I need a definition of the basic coding style. EditorConfig is an IDE
independent tool for this.
Use fixed versions for test dependencies.

These dependencies don't have to support a range because they are not
use by projects that depend on System Rules. Having a fixed version
makes the build more reliable because the version will not change with
a new release of a dependency.
RestoreSystemProperties restores all properties.

Simplifies usage of the rule. Users just need to paste the snippet.

    @Rule
    public final TestRule restoreSystemProperties
            = new RestoreSystemProperties();

and every property is restored.
Deprecated clearProperty(String).

This method is no longer needed because RestoreSystemProperties
restores all properties. Now the user can use System.clearProperty
in its test.