Use HTTP header for comments service auth

Previously, authentication was done using a URL parameter, which would
appear in various logs. Now, authentication is done more appropriately
with an HTTP header. Note that this requires cs_comments_service commit
cf39aabdd160176ebf206ca19d3ee030161a0b47 or later.
Merge pull request #4 from edx/gprice/cs-header-auth

Use HTTP header for comments service auth