Cleaning ext/nmatrix/ a bit

This commit doesn't change *any* behavior or feature. The modifications are as follows:

- Remove all the tabs from `ext/nmatrix/` files. (c'mon folks, configure your editors correctly)
- Fixed some compilers warnings, e.g. declaring some counters as `unsigned int` instead of `int`. Nothing dangerous.
- 80 characters line.
- I put braces on some one-line if-elses (remember that [Apple's SSL/TLS bug](https://www.imperialviolet.org/2014/02/22/applebug.html) earlier this year? It was due to a lack of braces...)

I'm working on some features and got surprised by these things, so I thought it'd be best to send this PR right now.