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.
Adapted birth response set prepoulator to new api. #3496
Adapted child and adhoc response set prepoulator to new api. #3496
Responses set prepoulation child classes should inherit from parent. #3496
Adapted non interview response set prepoulator to new api. #3496
Adapted participant verification response set prepoulator to new api. #3496
Adapted postnatal response set prepoulator to new api. #3496
Adapted pregnancy visit response set prepoulator to new api. #3496
Adapted tracing response set prepoulator to new api. #4596
Adapted pbs eligibility screener response set prepoulator to new api. #3496
Adapted low intensity response set prepoulator to new api. #3496
Implement skeleton helper methods in base response set prepoulator. #3496
Removing method which exists in parent class. #3496
Ordered methods to be identical to original response set populator. #3496
Fix incorrectly named class name. #3496
Adding missing method from old response set prepopulator. #3496
Add methods missed while adapting populator to new interface. #3496
Behavior of Person::start_instrument changed, so we need to make prepare_instrument work like before. #3496
Adapt birth prepopulator spec to use new prepopulator. #3496