Using emplace_* instead of push_*

[irwir]

emplace_* methods are more efficient when adding new objects, but might require C++11.
Does it make sense to add code for this?
Tests run successfully for the branch https://github.com/irwir/cryptopp/tree/use_emplace

[noloader]

Test the code with:

CXXFLAGS="-std=c++03" make -j 4

[irwir]

These flags are for GCC? I do not have it installed and almost always use Visual Studio, where currently the lowest possible seems to be /std:c++14
C++03 standards do not include emplace; it might be implemented but this could not be relied on.

[irwir]

This replacement could be done with a simple macro possibly named as CRYPTOPP_EMPLACE_BACK.
The definition may depend on CRYPTOPP_CXX11, or it could be a special CRYPTOPP_EMPLACE flag if necessary.

[irwir]

// emplace_front and emplace_back have been defined in C++11
#ifndef CRYPTOPP_CXX11_EMPLACE
#define CRYPTOPP_CXX11_EMPLACE 1
#endif

The idea is to add this definition into config_cxx.h at the end of CRYPTOPP_CXX11 block, and
after the CRYPTOPP_CXX11 block add CRYPTOPP_EMPLACE_BACK macro that expands to emplace_back or push_back method calls.

Is this good enough for a PR?