Ignore all hidden files

This commit adds an entry in .gitignore to exclude all files and
directories beginning with a dot.
Ignore certain entries only at the root level

This commit precedes some .gitignore entries with slashes
that were supposed to be only at the root level.
Replace boolean with CancelledState

This commit adds an enum CancelledState with two values, CANCELLED and
NOT_CANCELLED, which will be returned from methods dependant on
cancellable Bukkit events.
Make command executors final
Make AccountWatcher final
Rename local "e" to "ex"