Check for emplace member function support

cetlib and higher require presence of emplace member functions
in Standard Library collections. cpp0x does not provide a workaround
for these, so compilation can fail (known to affect GCC prior to 4.8).

Use new emplace tests provided by CheckCXX11Features to FATAL_ERROR
at CMake-time if these member functions are not available in std::vector
and std::map interfaces.

Add note in README to describe known minimal compiler/library support.
To be added to as time and knowledge progress.