Replace compatibility errors with documentation.

Leave compatibility stuff to Mechanize itself and just keep
http-cookie clean.
Improve rdoc.
Add tests for #path.
Improve tests for valid_for_uri?.
Conform to RFC 6265 5.1.4 in that path=/a matches /a/* but not /ab.

Remove HTTP::Cookie.normalize_path and add HTTP::Cookie.path_match?
instead for comparison.
Bump VERSION to 1.0.0.pre2.