HTTP::Cookie.parse: Change the signature again.

I made the uri parameter optional when I introduced the origin
attribute, but on second thought it should always be given.

I'm making the origin parameter fixed and mandatory again, but this
time it comes next to set_cookie.  This order should look more natural
because the one that comes first is to be parsed.

Since Mechanize::Cookie.parse required the uri parameter to be a URI
object, backward compatibility is still possible.
Add a utility shorthand method HTTP::CookieJar#parse.
Bump the version to 1.0.0.pre8.
origin=: Do not raise error if the new value is the same as the stock value.
Update rdoc.