Add clang++ compiler settings
Add g++ 4.6 compiler settings
Add g++ 4.7 compiler settings
Add g++ 4.8 compiler settings
Add MS Visual C++ 2013 compiler settings
Add MS Visual C++ 2013 Nov CTP compiler settings
Test on Travis CI
Implement Size feature
Implement Empty feature
Support features that requires at least one elemenmt in the TypeList

Add BaseTypeList and BaseHeadedTypeList, and specialize TypeList depending on
whether it has at least one element.
Implement At<Pos> feature