castling rights on rook capture. closes nvzqz/Sage#8

[captainsano]

No description provided.

[nvzqz]

This does not restore rook castling rights if the capture is undone.

Also, the if let _ = capture?.kind.isRook { part passes for any capture. Instead do:

if let capture = capture, capture.kind.isRook {
    ...
}

[captainsano]

Made the change with if let ... _undoMove seems to already restore the castling rights. What do you mean by undoing the capture?

[nvzqz]

Sorry, I should've specified. It's been a while since I looked through the code for the project. You're right. I forgot that castling rights were retained in history.

It also may look cleaner to use the isWhite and isBlack properties of Color.

One last thing, the tests on Xcode 7.3 fail to compile: https://travis-ci.org/nvzqz/Sage/jobs/185092484

[captainsano]

Done, build passing now!