Make rake db tasks work properly with sqlite when run not from the Rails root path.

The easiest way to encounter the problem is to create a fresh new engine and then run:

    rake db:create
    rake db:structure:dump
    Error: unable to open database "db/development.sqlite3": unable to open database file

So I fixed this for structure:dump and structure:load tasks in the same manner as it's already done for the drop task.