Allow rvalue_refs to be used with push and update methods (update, increase, and decrease)

[rcollyer]

Currently, only emplace has move semantics with regards to adding or updating the values in any of the heaps. This is limiting when the value_type is move only, especially as there is no workaround for the update methods. (emplace will invoke the move constructor, so it can be used as work around for push.) I propose adding overloads for the adding or updating methods to allow for values with move semantics to be used, e.g.

handle_type push(value_type&&);
void update(handle_type, value_type&&);
void increase(handle_type, value_type&&);
void decrease(handle_type, value_type&&);
void update_lazy(handle_type, value_type&&); // fibonacci_heap only

A similar issue exists in #23 for dealing with the comparison function.