exclude tests from being installed

Do not install `tests` as a top-level package -- such a package name
is bound to collide with other packages, and for this reason no package
should be installing files there.

Author: mgorny

setup.py

@@ -29,7 +29,7 @@ def readme():
     author_email='andrea.bonomi@gmail.com',
     url='http://github.com/andreax79/python-cstruct',
     license='MIT',
-    packages=find_packages(exclude=['ez_setup', 'examples']),
+    packages=find_packages(exclude=['ez_setup', 'examples', 'tests']),
     include_package_data=True,
     zip_safe=True,
     install_requires=[