Add support for 64 bit architectures

A few minor changes were required to squash the compiler warnings when building for arm64. All warnings were generated by the new rules around `GCC_WARN_64_TO_32_BIT_CONVERSION`.

- Abstract `ceil`, `floor`, `round` method calls with something that switches on the current architecture
- Encode NSNumbers rather than float values within `encodeWithCoder:`

The only other change is to the demo project to enable `ARCHS_STANDARD_INCLUDING_64_BIT` which may not be desired, and updating the README to reflect the arm64 support.