Standalone versions of nbgrader assign, autograde, and feedback

[jhamrick]

Fixes #763

This creates standalone objects that can be used for assigning, autograding, and feedback. For example:

from nbgrader.coursedir import CourseDirectory
from nbgrader.converters import Assign
from traitlets.config import Config

import logging
logging.basicConfig(level="INFO")

c = Config()
c.CourseDirectory.assignment_id = "ps1"
c.Assign.create_assignment = True
c.Assign.force = True

coursedir = CourseDirectory(config=c)
converter = Assign(coursedir=coursedir, config=c)
converter.start()

which will produce the expected nbgrader assign output:

WARNING:traitlets:Removing existing assignment: /private/tmp/course101/release/ps1
INFO:traitlets:Copying /private/tmp/course101/source/./ps1/jupyter.png -> /private/tmp/course101/release/./ps1/jupyter.png
INFO:traitlets:Updating/creating assignment 'ps1': {}
INFO:traitlets:Converting notebook /private/tmp/course101/source/./ps1/problem1.ipynb
INFO:traitlets:Writing 7167 bytes to /private/tmp/course101/release/./ps1/problem1.ipynb
INFO:traitlets:Converting notebook /private/tmp/course101/source/./ps1/problem2.ipynb
INFO:traitlets:Writing 1541 bytes to /private/tmp/course101/release/./ps1/problem2.ipynb
INFO:traitlets:Setting destination file permissions to 644

This is a first step in getting #555 working.

[willingc]

Fantastic!