Provide Mechanize::Cookie/CookieJar for compatibility.

This is achieved by prepending compatibility layers to HTTP::Cookie
and HTTP::CookieJar.  Those layers are made to emit a warning (only
under $VERBOSE mode) every time an obsoleted method is called or a
call to a renewed method is given an obsolete set of parameters.
Bring back tests for Mechanize::Cookie and Mechanize::CookieJar.

Note that they don't pass yet due to minor changes between the old
cookie modules and the new ones.
Add CookieJar#jar to the compatibility layer.
Require http-cookie ~> 1.0.0-pre2.
The new parser rejects time zone offset in accordance with RFC 6265.
The new parser rejects numeric month in accordance with RFC 6265.
The new parser handles double quotes in accordance with RFC 6265.
The new parser rejects a cookie with a domain different from the origin.
CookieJar#save_as should not save session cookies by default.

This test and the original implementation are wrong.
A leading dot of the domain field value is optional.

The new saver does not add one, so fix the test.