Add getHeaders() method + parsing headers logic

[flarive]

Hey !

I have added a getHeaders() method to be able to easily retreive headers data from FBX file (app, creator, FBX version...)

Here is how i use it in my own app that uses openfbx :

void fbx_mesh_loader::get_metadata(const ofbx::IScene* scene)
{
    const ofbx::Headers* headers = scene->getHeaders();
    if (headers)
    {
        int fbxHeaderVersion = headers->fbxHeaderVersion;
        int fbxVersion = headers->fbxVersion;
        int encryptionType = headers->encryptionType;
        char* creator = headers->creator;
        char* creationTime = headers->creationTime;

        char* documentUrl = headers->documentUrl;
        char* srcDocumentUrl = headers->srcDocumentUrl;

        char* originalApplicationVendor = headers->originalApplicationVendor;
        char* originalApplicationName = headers->originalApplicationName;
        char* originalApplicationVersion = headers->originalApplicationVersion;
        char* originalDateTimeGMT = headers->originalDateTimeGMT;
        char* originalFilename = headers->originalFilename;
        char* originalApplicationActiveProject = headers->originalApplicationActiveProject;
        char* originalApplicationNativeFile = headers->originalApplicationNativeFile;

        char* lastSavedApplicationVendor = headers->lastSavedApplicationVendor;
        char* lastSavedApplicationName = headers->lastSavedApplicationName;
        char* lastSavedApplicationVersion = headers->lastSavedApplicationVersion;
        char* lastSavedDateTimeGMT = headers->lastSavedDateTimeGMT;
    }
}

FBX datetime are returned as strings by my code, it could be improved but everything works well.
I can make changes or fixes if you want me to.

Hope you'll merge that in master !

Eviral