added instrumentation to count: user logins, registrations, correct answers, wrong answers, enrollment

- give the preview div an id, use that to look it up instead of dancing around in the DOM

- make chemical_equations_equal return False instead of raising errors.
- make tests not choke on unicode
- get full coverage

- architecturally slightly questionable: the preview ajax calls goes to an LMS view instead of an input type specific one.  This needs to be fixed during the grand capa re-org, but there isn't time to do it right now.
- also, I kind of like having a generic turn-a-formula-into-a-preview service available

- for now, wraps existing render functions as separate classes
- a bit of cleanup in how it's called from capa_problem
- initial tests to make sure things are testable.

- give different errors when student or author stuff is messed up (I was seeing "No way to interpret '3' as a number" :P)
- preserve traceback in exception

* Move tests into a separate file
* add a chemical_equations_equal function to compare equations, not expressions
* rename some internal functions with a leading _