Move filetree to file_tree to unify file names.

src/lib.rs

 
 mod database;
 mod file_system_watcher;
-mod filetree;
+mod file_tree;
 
 pub use database::Database;
 pub use file_system_watcher::FileSystemWatcher;
-pub use filetree::FileTree;
+pub use file_tree::FileTree;
 
 // Sketch of the synchronization algorithm:
 //