Correcting header guards

See https://www.securecoding.cert.org/confluence/display/cplusplus/DCL32-CPP.+Do+not+declare+or+define+a+reserved+identifier#DCL32-CPP.Donotdeclareordefineareservedidentifier-NoncompliantCodeExample%28HeaderGuard%29
Merge pull request #6 from damiancarrillo/header_guards

Correcting header guards