Implemented a nick compare function

As suggested by the todos in IRCSock, added IsNick(CString) method
so the ugly Nick.GetNick().Equals(GetNick()) could be simplified.

Signed-off-by: Toon Schoenmakers <nighteyes1993@gmail.com>

include/znc/Nick.h

@@ -37,6 +37,7 @@ public:
 	void Parse(const CString& sNickMask);
 	CString GetHostMask() const;
 	size_t GetCommonChans(std::vector<CChan*>& vChans, CIRCNetwork* pNetwork) const;
+	bool IsNick(const CString& nickname) const;
 
 	// Setters
 	void SetNetwork(CIRCNetwork* pNetwork);