Using some C++0x features
- Core.h: using C++11 static_assert for staticAssert when possible
- fixed warnings in C++11 mode of GCC:
  - concatenation of strings - can't use "str1""str2", but use "str1" "str2" (needs a space between quotes) - see changes with enabled whitespace differences
  - RGBA macro - use 'unsigned' instead of 'int' to avoid warning about conversion of negative int to unsigned
- added "-std=c++0x" option to GCC (required for static_assert)