Support specifying user and password in repository

[marianopeck]

Right now there is no way to specify the username and password of a given repository. Not even this works:

Metacello new configuration: 'Quuve'; repository: (MCHttpRepository location: 'http://smalltalkhub.com/mc/AAA/BBB/main' user: 'X' password: 'Y'); version: '1.7'; get; load.

Note that AFTER you have loaded something specying a username and password it seems it is remembered. I think the behavior should be the following:

1. If username and password is specified to "Metacello new" then THOSE must be used, no matter if there were already some cached or remember user/pass.

2. If none user/pass is specified, then allow fallback to the stored/cached one (if exists)

[LinqLover]

See #536 (WIP) (and probably #536 as a workaround). Could this be helpful for you? :-)