cereal v0.9.1

This release introduces a few new features which can be found in detail here. In summary, they are:

• Making archives more portable by switching away from size_t internally
• Adding a portable_binary.hpp archive that handles little vs big endian machines (NOTE: this still needs more testing across architectures. It passes unit tests and simulated endian swaps)
• GCC 4.7.3 is supported as the minimum GCC version
• Supports compiling using libc++