Added buckets for diagonal slices.
Added placeholder functions which will convert SAN notation to forward and backward diagonals.
Finished initial implementation of _initialize_board function.
Added sets of cached moves to the piece class.
Modified diagonal bucketing so that all diagonals are named for their bottom-most cell.
Implemented _san_to_backward_diagonal.
Added empty test files.
Added empty exceptions file, so that eventual exceptions can be encapsulated separately.
Added skeleton tests for the conversion of spaces to the diagonals in which they have membership.
Added initial test for forward diagonals (needs more work).