Code cleanups

[mathieucarbou]

• Typo
• String instead of fixed buffers for username/password
• setAuthentication(const String& username, const String& password)
• Bug in setAuthentication logic : empty strings were not checked
• Moving internal macros and enum in CPP file

[mathieucarbou]

@ayushsharma82 : please review carefully because I found some weird things... Not sure how it was working.