stelar Client initialization

[vsamtuc]

To make Client credential instantiation more extensible in the future, implement

Client(url, credentials)

where the second argument can be one of several things:
(a) the current impl: { 'username': ..., 'password'; ... }
(b) a context-based mechanism (similar to k8s contexts: a file stored in $HOME/.stelar containing
the username/password for each URL that the user has account on.
(c) other, e.g. the JWT directly, to serve uses of Client inside web apis, etc..