Moved some strings to a simple header file.

  This is all I'm doing for issue #18 at the moment, if 7thisle
 can move to something better I'd be pleased.

  We should probably look at one of the libraries for making this
 hard as some of the strings I've not moved have arguments.  e.g:

          printw("No maildirs found matching the limit '%s'.", limit->c_str());