added dependency info to README
Column: added copy and move ctors/assign operators

Also m_handler is now of std::shared_ptr type.
Tab: added copy&move ctors/assign operators

+ new Type_factory::Type_product signature
TypeFactory: ctor now takes a const ref to Type_product
Type_factory: Type_product now uses a new signature

* added a default ctor
* copy dtor is now disabled
* register_type now takes Type_product as a const reference
dir: making use of get_handler_hash<> function
List_dir: changed ctor

There's no need to supply type because List_dir already knows its type.
hash&dir_hash: added hashing template functions

These are used to calculate handler hashes (and are stored in a static
variables so they're calculated only once).
Cache: added a move ctor and a move assign operator
Handler: added move&copy ctors&assign operators

Also a virtual destructor has been added (it's absence caused an obvious
memory leak).