Switch LMS over to using XBlock rendering commands

This makes the LMS use an XBlock's student_view, rather than an
XModule's get_html to render for display. However, it does not yet use
wrap_child to handle instructor debug information or url rewriting.

[LMS-219]
Merge pull request #487 from edx/cale/lms-xblock-student-view

Cale/lms xblock student view