Allow more contents manager functions to return futures

This allows slower contents managers to not block the event loop by allowing
more of their API to return futures.

Other usages of contents manager functions are already wrapped in maybe_future,
including a use of `file_exists` in contents/handlers.py

notebook/files/handlers.py

@@ -12,7 +12,7 @@
     from base64 import decodestring as decodebytes
 
 
-from tornado import web
+from tornado import gen, web
 
 from notebook.base.handlers import IPythonHandler
 
@@ -51,7 +51,7 @@ def get(self, path, include_body=True):
         else:
             name = path
 
-        model = cm.get(path, type='file', content=include_body)
+        model = yield gen.maybe_future(cm.get(path, type='file', content=include_body))
 
         if self.get_argument("download", False):
             self.set_attachment_header(name)

notebook/services/contents/handlers.py

@@ -182,10 +182,12 @@ def post(self, path=''):
 
         cm = self.contents_manager
 
-        if cm.file_exists(path):
+        file_exists = yield gen.maybe_future(cm.file_exists(path))
+        if file_exists:
             raise web.HTTPError(400, "Cannot POST to files, use PUT instead.")
 
-        if not cm.dir_exists(path):
+        dir_exists = yield gen.maybe_future(cm.dir_exists(path))
+        if not dir_exists:
             raise web.HTTPError(404, "No such directory: %s" % path)
 
         model = self.get_json_body()