Add support for callbacks with no argument

[pjueon]

related to #53

Changed:

• Add support for callbacks with no argument

[pjueon]

Need to fix:

• If the callback object can be called not only with a const std::string& argument and also without any argument, the method without any argument will be called in the current implementation.

// if you use this object as a callback, 
struct Foo
{
    void operator()();           // this method will be used as a callback function in the current implemantation
    void operator()(const std::string& channel);
};

• I think it would be more natural to use the method with a const std::string& argument.
• It should be stated in the document that which method will be used in this scenario.

[pjueon]

Need to fix:

• If the callback object can be called not only with a const std::string& argument and also without any argument, the method without any argument will be called in the current implementation.

// if you use this object as a callback, 
struct Foo
{
    void operator()();           // this method will be used as a callback function in the current implemantation
    void operator()(const std::string& channel);
};

• I think it would be more natural to use the method with a const std::string& argument.
• It should be stated in the document that which method will be used in this scenario.

Fixed the issue.