Immutable BigNumber

[rande]

Hello,

I would like to use this library on the sonata-project, however the BigNumber class is mutable, I would like to introduce a new ImmutableBigNumber class.

What do you think ?

[ramsey]

The BigNumber class is mutable because it has mathematical operations that may be performed against it, changing its value. What are you thoughts on how the immutable version of the class would work?

[rande]

see #2