Added support for Azures Client secret

[Cuddliness]

Since i was not able to get a refresh token anymore, due the need of a client_secret. I added the support for that :).

Why?

I was not able to login with a refresh token, since it asked for a client_secret.

MsaAuthenticationService service = new MsaAuthenticationService("AzureID", "AzureSecret");

[Cuddliness]

Pushed new code where the client_secret is optional,

Maded changes on each request that can use a client_secret, the requests will only include a client_secret when this is set.
As stated above you can now use it as:

MsaAuthenticationService service = new MsaAuthenticationService("<clientID>");
            service.setClientSecret("<secret>");
            service.login();

PS: it's my first ever contribution so i appricate the feedback!
Let me know if something can be diffrent / better!