Add Travis CI test configuration

This will run the tests on CPython (2.5-3.3) and PyPy. Each push will
automatically trigger a test run on the Travis CI server.

.travis.yml

+language: python
+
+python:
+    - "2.5"
+    - "2.6"
+    - "2.7"
+    - "pypy"
+    - "3.2"
+    - "3.3"
+
+install:
+    - python setup.py install
+
+script:
+    ./tests/tests.sh