Custom 404 Mixin to allow for a particular 404 template to be rendered per view.
Merge branch 'develop' into feature/custom_404_handler
Merge branch 'develop' into feature/custom_404_handler
Tests for Custom404Mixin.  Some other pep8 fixes in same file.
Added section to application's README that gives information on usage of the Custom404Mixin.  Split tests for mixin into a separate test file by classname.  Minor PEP8 formatting fix in mixin.
Merge pull request #2 from Harvard-University-iCommons/feature/custom_404_handler

Feature/custom 404 handler