ENH: Add warning when colname collides with methods

Current behavior does not allow attribute-like access when the column
name of a DataFrame matches the name of a method. This commit adds an
explicit warning about this behavior. Closes pandas-dev#5904.

pandas/core/generic.py

@@ -1787,6 +1787,10 @@ def _slice(self, slobj, axis=0, kind=None):
         return result
 
     def _set_item(self, key, value):
+        if callable(getattr(self, key, None)):
+            warnings.warn("Pandas doesn't allow attribute-like access to "
+                          "columns whose names collide with methods",
+                          stacklevel=3)
         self._data.set(key, value)
         self._clear_item_cache()