Migrate mass-assignment security to strong_parameters

Ahead of the upgrade to Rails 4, this change removes the use of
attr_accessible and attr_protected as mass assignment protection
mechanisms in favour of whitelisting input attributes at the
controller level.

See http://blog.markusproject.org/?p=5588 for more details.
Handle submission rule changes correctly in assignment editing

Resolves GH-1622.
Conversion of README to Markdown
Fix section due date duplication

They were not being properly whitelisted previously, and the
workaround did not check if the due dates already existed before
creating new due dates.

This fix allows Rails to simply Do The Right Thing.
Make the section dues message less awkward
Fix a Hound warning
Add awesome_print to development mode

To help print out database records in rails console nicely (sanely).
Update dependencies
Reimplement students table in React
Add error handling and fix selection bug
Replace admins table in React
Replace tas table in React
Replace groups manager in React
Add 'add new group' button to groups table
Add ability to pass DOM object for sortable columns
Create SelectTableFilter for dropdown filter
Replace non-criteria tables in React
Separate GraderManager into files
Finish template for adding tabs w/ criteria table
Complete framework for graders table w/ crit