Add BLAS i?amax function

The old one, `idamax`, wasn't using BLAS's functions when possible.
Besides, it was the wrong name! According to the specification, the
correct name is ixamax (where x \in {f,d,c,z}).
Using imax instead of idamax
Return a Ruby Fixnum

This commit also removes some compilation warnings. (yay)
Add test for NMatrix::BLAS.cblas_imax
Add documentation to NMatrix::BLAS.cblas_imax
Merge pull request #231 from agarie/blas_imax_imin

Make NMatrix::BLAS.cblas_imax call BLAS functions