This patch handles nil values when passed into the slice argument
of AppendNonNil(err error, errs ...error). nil values are filtered
out and not included in the multierror. When the first argument
to AppendNonNil is nil, and the second argument consists entirely
of nil values, then return a nil value.

This patch allows the usage the following style.

```
var result error

result = multierror.AppendNonNil(result, step1())
result = multierror.AppendNonNil(result, step2())

return result
```

result will be nil if-and-only-if step1() and step2()
both return nil.

Do not convert 'nil' error to 'nil' *Error.

When the filtered errs list has zero length then preserve
the input error.

Change root path to fork

Clarify README.md documentation.