Update to RSpec 2.12.  #3496.

I want to be able to use parameterized shared contexts in my response
set populator tests.  (In my case, the parameter is the name of a test
survey that will be loaded in a before-all.)  This capability doesn't
exist in RSpec 2.6.

Said capability was probably introduced sometime before 2.12, but I've
been using 2.12 for a while and it seems stable enough, so might as well
just use it.
New response set prepopulation pattern.  #3496.

There are no specs in this commit, and therefore I have no idea if this
works.  Topic branches, etc.

The new prepopulators will be filled in from existing code in future
commits.
Start Mustache context prepopulator.  #2744.
HandlebarsHelpers is a better name.

The Mustache context is the context in which the helpers will be
evaluated.
InstrumentContext no longer needs to derive from Mustache.  #2744.