Add a pair class

[eyalroz]

Yes, you knew it had to happen at some point... one of the ugliest, wartiest class in C++, which you would have like to just replace with:

template <typename T1, typename T2> struct pair { T1 first; T2 second; }

and be done with it... we need to implement an std::pair equivalent. If for no other reason, then for #18, as some of the standard algorithms use pairs.