Adds segment-io integration into the LMS

Tell segment IO whenever a page is loaded.  Once this works, we
can add more detailed instrumentation for events we care about.

Will be turned off in prod until the SEGMENT_IO_LMS feature flag is
set to True and a SEGMENT_IO_LMS_KEY is set.
Merge pull request #65 from edx/victor/segment-io

Adds segment-io integration into the LMS