Change name of test_capture to test_move_capture

That way, it doesn't conflict with board's test of the same name, so I
get all of tests executed.
Introduce movegen.

Add initialization methods and provide a king_movelocs array indexed by
location.
Add Ruby-facing methods for king_movelocs

This probably should be taken away later; I don't see any point to
giving Ruby access to this method. It's here pro tempore for testing's
sake.
Add minimal testing for kingmoves

I should probably add more tests to deal with the other corner cases
(excuse the pun), but this'll do for now (I hope).