Make better use of C++11 features

[yeputons]

This PR is based on #3, which is scheduled to be merged after the course ends. I believe this PR's fate is going to be similar.

Things done:

• Replace pure pointers with std::unique_ptr. If C++14 is allowed, some boilerplate can be replaced with std::make_unique. This also enables the full power of RAII and removes a lot of destructors.
• Replace NULL with nullptr.
• Mark overrides of virtual functions with override keyword instead of virtual (override implies virtual).

Things that can be improved:

• Better use of move semantics. E.g. LSelectNode(BaseTable& table, std::vector<Predicate> predicates); always copies its second argument, even if it's a temporary value. If one replaces this->predicates = predicates; with this->predicates = std::move(predicates);, it will use move instead of copy in these situations.

[chernishev]

Thanks.

I dislike std::move, because its quite easy to get default copying behavior without any warning, if you do something wrong. Also, using std::move requires a bit of additional C++ understanding from a student, but this is course about databases (and it is already complex), not C++ programming.

Override and nullptr are good, other students already mentioned it. About uniques I will think, I recall that there were some considerations against having them in this project (aside from providing Valgrind playground).