staff grading xmodule - initial version MVP / WIP

Provides staff grading capability, via an xmodule.  Files are uploaded to an S3 bucket (using the same config variables as for `combinedopenended`).  Metadata is stored in a SimpleDB domain ("`staff_grading`").  Staff who view the staff grading problem are given an option to see all the uploaded files, and to enter grades and comments.  

Files can be uploaded until the due date, or until a grade is assigned.

Example XML and screenshot:

    <sequential display_name="Staff Grading Tests" url_name="Staff_Grading_Tests">
      <vertical url_name="Staff_Grading_Tests_vertical">
        <staffgrading display_name="Staff Grading Example" the_max_score="50" />
      </vertical>
    </sequential>

![image](https://f.cloud.github.com/assets/1590761/2062311/be778caa-8c85-11e3-8c41-54e6c9fad4bb.png)

This is a MVP implementation, but a WIP.

Still to be done:

* Limit filetypes for upload
* Paginate page showing table of uploaded files
* Check to make sure grades are stored properly and processed by the grader
* Fix icon to use "progress" semantics
* Allow staff to upload annotated PDF files, which can then be downloaded by students
* Improve javascript, to use non-global definitions
* Clean up imports and pylint
* Add tests
* Add approval step for grades (e.g. so TA's can grade, and instructor approves)