OSX: don't try to call *nrm2 blas calls if incx < 1 or N < 1

Because calling nrm2 with negative incx will lead to a crash on
OSX. See issue #37.
OSX: Use cblas interface for *dot blas functions

Using the documented cblas_*dot interface for the *dot BLAS
functions on OSX (using the Accelerate framework) will give the
expected result in contrast do using *dot_ functions.
OSX: Use cblas interface for *asum blas functions

Using cblas_*asum instead of *asum_ will give the expected result
Add an comment to explain the nrm2 workaround on OSX
Merge pull request #39 from gicmo/nrm2osx

[OSX] Fix crash in nrm2 tests