Add schoolyourself-xblock to edx-private.txt. This is for use by AlgebraX and GeometryX running on edX.org in Q1 2015.

Fix a typo in the comment for the schoolyourself-xblock requirement

Update schoolyourself-xblock requirement to the latest version

Update the link to schoolyourself-xblock to point to the latest version, which removes hmac from requirements.txt
Merge branch 'schoolyourself_xblock_requirement' of github.com:schoolyourself/edx-platform into schoolyourself_xblock_requirement