Test for C++11 string numeric conversion support

Implement compile/run test for presence and functionality of
numeric conversion functions in <string> header. Only exercise
std::stoi and std::to_string, though these should be sufficient.
Implement run fail test of invalid_argument exception throwing when
passing std::stoi an unconvertable string. If tests pass, set
HAS_CXX11_STRING_NUMERIC_CONVERSIONS CMake variable. Document this
variable in the CheckCXX11Features module, and link it to the
corresponding FNAL cpp0x definition in the README.