Installation bug fix

setup.py was unconditionally importing pytz, which is a dependency; but
setup.py must not import any dependencies (otherwise it tries to import
them before installing them), so this import was moved to the relevant
section of the file.
Updated CHANGES.txt
Minor code style issue
Removed spatial from py2exe

Since we don't support a Windows installer with bitia included, we
remove spatial from py2exe so that it can do its work on a generic
Python installation instead of OSGeo4W.