don't require cython to install
fix eastern hemisphere longitudes

This commit is aimed to address #7.

Changes:
- while parsing XML-like metadata, checks for sign in strings
- sets `flip_lon` default to False
convert coords from eastern hemisphere correctly

Previously, irlib assumed coordinates from the western hemisphere, because the
HDF coordinates were unsigned. It seems that this has been addressed, so this
turns off the code that forced negative longitudes.

I'll need to dig up some old IceRadar HDF files to see if those fail now. If so,
maybe I can come up with a heuristic based on the GDF metadata version.

Fixes #7