Getting draft split_test_module instances to import/export.

STUD-327
Added text about grade ranges and certificates
Merge pull request #4529 from edx/markhoeber-documentation-doc420

Added text about grade ranges and certificates
Merge pull request #4491 from edx/christina/export-split

Getting draft split_test_module instances to import/export.
Merge pull request #4531 from edx/sylvia/docs/relnotes_7_23

Release notes for 7-23-2014