Add new documentation section for integrators.
Remove isfinite from std_real_concept.hpp - it's not part of our conceptual requirements.
Add workarounds for older compilers that can't handle the default arguments to the function.
Restrict testing test_trapezoidal.cpp to compilers that handle lambdas.
Make the concept checks C++03.
Make test_trapezoidal.cpp compile with VC11's broken lambda support.