support multi directory search in XToDoModel.m

now,  data modal can add some dirs to search for, and add some dirs to
exclude, the next thing todo is : finish UI
Support search in multi directories

Support search in multi directories, exclude multi directories, the two
path settings are related to project name, save setting in path
“~/Documents/XTodo/%ProjectName%.plist”
show progress indicator while searching TODO items

Searching work has moved to asychronous thread，
Make sure temp file was deleted

Move temp file form NSTemporaryDirectory to “~/Documents/XTodo/Temp/”,
clean all files in this directory while starting up
Tags setting did not work
Merge pull request #28 from shuice/master

custom search directory in preference window.