Added command-line scripts to setup.py

This implements #85. I used the entry_points way of setuptools because it is directly compatible to Windows (generating .exe files on install). For that I had to copy the scripts into a nansat package (nansat.cli for command-line interface).

There is also a small unrelated change in here for _ask_gdal_config() such that exceptions aren't caught in this function but when actually using it (now it actually works with the fall-back of not compiling pixel functions...).