Implement pluggable SASL handlers in strophe

[Olemis Lang]

After reviewing strophe.js (within the body of _connect_cb function) I've found that vars do_sasl_digest_md5,
do_sasl_plain and do_sasl_anonymous are used to select the SASL auth
mechanism that should be used .

Since it's possible to have custom auth mechanisms, IMO it would
be nice to have methods like PyXMPP's challenge which would make
strophe more flexible & easy-to-extend . I mean , the actual code
looks like a well known anti-pattern , and it would be nice to be able
to register «auth handlers» and select the correct match considering
the value of mech var .

IOW do something like

auth_mechs = {
 'DIGEST-MD5' : handle_DIGEST_MD5,
 'PLAIN' : handle_PLAIN,
 'ANOMYNOUS' : handle_ANONYMOUS,
}

auth_handler = auth_mechs[mech] || handle_no_auth_mech;

something = auth_handler(...)

... in that case it would be possible to add custom auth without
patching strophe.js by executing auth_mechs['MY_METHOD'] = handle_MY_METHOD similar to what is possible in PyXMPP by
implementing and registering ClientAuthenticator subclasses .

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please be so kind and make a pull request :)