Remove some compiler warnings

I mostly added some type casts for unsigned integers and such.
Updated README

I forgot to add the IxAMAX BLAS function to the README and there was an
outdated comment regarding documentation.

This commit also removes a lot of extra whitespaces in `ext/nmatrix/`
files.
Removed curly braces in if statement

As per John's request.
Merge pull request #233 from agarie/compiler_warnings

Remove some compiler warnings