Add continuous integration with travis-ci, first preview

I like the testnet box and though about running my tests against it. One of the reasons behind this was that I wanted also to add continuous integration tests with travis-ci to bitcoin-python.

I thought the best way to show my idea was to do a first automated test run. The branch is travis, https://github.com/nkuttler/bitcoin-python/tree/travis. The output of the first successful automated test run is at https://travis-ci.org/nkuttler/bitcoin-python. Obviously the code needs to be refactored, it's just to show the basic idea.

I host a testnet box on a server to avoid installing it in every travis-ci test run, the RPC access configuration is encrypted in the .travis.yml file (http://about.travis-ci.org/docs/user/build-configuration/#Secure-environment-variables).

With this approach the tests can spend as many coins as they like, and resetting the testnet box is possible. It could be reset it daily, weekly, or whatever is appropriate. If you like this approach I'll also add documentation for all the necessary steps.

I wasn't sure if I should already make a pull request, but this seemed like the best way to start a separate discussion.