Factored out the _is_marking() function from the memory datastore
module to utils so it can be reused, and changed both filesystem
and memory datastore modules to import and use it.
Don't wrap objects in a Bundle when adding them to a FileSystemStore,
but still support getting objects from FileSystemStore that were saved
in a bundle.
Also:
- Add option to allow custom content in FileSystemStore
- Simplify an if statement
- Improve FileSystem docstrings
- Remove an unnecessary parse() call in FileSystemSource.get()
Chris Lenk