Add support for hexadecimal byte numbers

[bvssvni]

Some formats store the hexadecimal byte representation of numbers to avoid loosing information. For example 0x3F800000 is 1.0.

The motivation for this is to support the full OpenGEX specification.

[bvssvni]

Could be added as a h rule for "hexadecimal" to the meta language.

[bvssvni]

Alternatively, this could be added to NumberSettings in read_token library.

[bvssvni]

Remembered something I've read about the Witness blog: http://the-witness.net/news/2011/12/engine-tech-concurrent-world-editing/