Aleth's databases should be stored in an "Aleth" subdirectory

[halfalicious]

Aleth's databases are currently created in $HOME\.Ethereum on *nix and %APPDATA%\Ethereum on Windows. Geth creates its databases in $HOME\.Ethereum\geth and %APPDATA%\Ethereum\geth. Aleth's databases should be created in an "Aleth" sub-directory so that when one runs multiple clients it's easier to delete the Aleth databases without affecting other clients.