Added HeadingInfo.Default and CopyrightInfo.Default.

HeadingInfo.Default exposes the default heading generated by
HelpText.AutoBuild.
Sensible defaults have also been added, removing the need to explicitly
add assembly attributes (though they still take precedence).

CopyrightInfo.Default exposes the default info generated by
HelpText.AutoBuild.
Similar to HeadingInfo.Default.

Also fixes bug #22.