`separate_header` kwarg option in `write`

This PR includes changes necessary to allow the `write` function to create separate .nhdr (header) and .nrrd (raw data) files.  It is an optional argument, turned off by default, and should not break any legacy code.  I created this because I needed a way to both import and export this format with separate .nhdr headers, and am pushing it back in case others may find it useful.

My editor made a few other automatic changes regarding PEP8 and line lengths, as well as `import numpy as np`, and there was an error in the last line of nrrd.py (`formatted_fields` got renamed `options` at some point in development, but that didn't get updated).

My tests verify proper writing and reading on Scientific Linux 6.2, Python 2.6.6, for any set of arguments passed to `write`.