Implemented optional asynchronous authentication

[schantaraud]

Changes:

• The server.authentication function can either return a value or call an optional callback to allow for authentication from databases or other webservices. If both are used, the first one wins, the second is ignored.
• In case of authentication error, a SOAP Fault is returned with a status code of 401 (previously: standard exception with optional stacktrace, which soapClient doesn't like).

All existing tests results unchanged, new tests: test/server-authentication-test.js.

All feedback is welcome.

[coveralls]

Coverage increased (+0.1%) to 93.714% when pulling 92ec3c4 on schantaraud:master into 7a5550a on vpulim:master.

[coveralls]

Coverage increased (+0.1%) to 93.714% when pulling 92ec3c4 on schantaraud:master into 7a5550a on vpulim:master.

[jsdevel]

@schantaraud the node 4 tests are failing. can you take a look?

[jsdevel]

nvm. node 4 is dead