Support for configurable time formats

[alexejk]

Closes #100.

<dateTime.iso8601> was hardcoded to time.RFC3339 in both directions, which doesn't match what servers actually emit — the spec shows a compact example, mandates no layout, and leaves timezone assumptions to server documentation.

A new TimeFormat option makes encoding and decoding configurable:

c, err := xmlrpc.NewClient(endpoint, xmlrpc.TimeFormat(&xmlrpc.LayoutTimeFormatter{
    FormatLayout:   xmlrpc.LayoutISO8601Compact, // 19980717T14:08:55
    FormatLocation: time.UTC,                    // zone-less layout needs a pinned zone
    ParseLayouts:   xmlrpc.CommonParseLayouts(), // accept the common variants
}))

Fields come in two pairs: Format* controls what goes on the wire, Parse* what is accepted off it. Servers doing something stranger can implement the TimeFormatter interface directly.

Defaults are unchanged. With no option set, values encode and decode exactly as before, including time.Parse's handling of offsets that match the local
zone. The one observable difference is that a failed default parse now returns a wrapped error rather than a bare *time.ParseError — errors.As still works, a direct type assertion does not.

Thanks to @stricker-devmode for raising this and for the analysis in #100.

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