Updated Signal.hpp - added fix for C++11

[inobelar]

For making psychic-ui::signals much portable (in case of standalone usage) - added small fix to make it compilable with C++11.

In Signal::unsubscribe(), inside of std::remove_if(), in lambda used 'auto' item type. That's not allowed in C++11. Used explicit type. Also added 'const qualifiers' where them needed. Also added missing #include for std::shared_ptr<T>