Put rust-bignum on crates.io !

[yvan-sraka]

Why not upload this repository on https://crates.io/, I need a rust wrapper for the GMP library, and this one, even a bit old, seems to do the job!

If you advise using another package to do this stuff (which will explain why it is not available with cargo install) I think then it would be cool to mention it in README.md.

[jsanders]

Sorry I just noticed this! This project predates cargo and I haven't done anything with it since cargo came around :) I saw your PR to fix some build errors (sorry, I won't be able to look at it today, but I'll try to get to it soon.) Do you think after that, this would be suitable for crates.io as-is, or would I need to do some more fixing up?

[yvan-sraka]

Other fixes are needed, but I've worked on most of them and if you have little time to review my work, I will open other PRs :)