Run tests and code checks on Travis CI

![Travis CI](http://about.travis-ci.org/) is a continuous integration project for open-source projects. It shows whether the build passes or fails due to some error on each commit. Also it shows nice status for pull-requests - either they break tests or not.

This branch does the following:
 * Installs requirements
 * Runs ```pyflakes``` to eliminate possible errors in the code (e.g. missing imports, mistyped variables etc.)
 * Runs ```pep8``` check to ensure that code style is not broken (thus closing issue #37)
 * Runs self-test (```scripts/mailpile.py```) to make sure that changes didn't break anything
All of those steps are repeated for supported python versions (2.6-3.3) so we can see if the change broke 2.6 or incompatible with 3.3 etc.)

Currently travis will show many errors, but it would be easy to identify failing parts using pyflakes + pep8.

This script can be extended:
 * Build debian/ubuntu/whatever package and upload it to repository
 * Run other scripts/tests
 * Verify HTML/JS
 * Add coverage collection and display it via ![coveralls](http://coveralls.io)