add note on request for flat project tree view
add more checks for eclim sign level

vim-airline's eclim extension makes calls to
eclim#display#signs#GetExisting which can impact performance when called
on ever status line update. So, if the user sets g:EclimSignLevel = 0,
then don't even parse the existing signs, just return an empty list. Add
a few other checks as well to disable placing/viewing user signs.

fixes #304