Fixed collision bug that was especially visible at lower framerates.

problem: when moving in two dimensions (not in a straight line) you
could get stuck inside blocks.

cause: the d_t fix made moveX and moveY functional, moveY's result
didn't depend on moveX result. This caused a problem because then moveY
didn't take into account the horizontal axis and that's why you got
stuck.

fix: made moveX and moveY take a Vector as argument instead of just the
x respective y position. result of moveX is stored and passed into moveY
to get correct results.

reflection: A very imperative approach. Can perhaps be reworked to be
more concise and readable.
made friction and accerelation in Entity respond to dt.
Also made a limit to dt