Travis yml

As brought up on [pytables-dev](https://groups.google.com/forum/?fromgroups=#!topic/pytables-dev/sRHDFpreAYk), enabling http://travis-ci.org for PyTables may be useful both for internal and external devs. This branch adds a .travis.yml based almost entirely on our [jenkins/hudson](hudson.openmicroscopy.org.uk/job/PyTables) job. 

I enabled travis-ci for all of my branches by logging in via github and going to https://travis-ci.org/profile. After that, ever time I push a branch on an enabled repository, travis builds it. (See https://travis-ci.org/joshmoore/PyTables/builds/4824380)

Once the main repository has a .travis.yml file (i.e. once this PR is merged), we can enable the main repo (https://travis-ci.org/PyTables/PyTables) and not only will every pushed branch be built, but also every opened PR.

An indicator (https://github.com/blog/1227-commit-status-api) will be added to all tested commits. (This is **very** nice).

We can also then add a status image (https://travis-ci.org/joshmoore/PyTables.png) to our README.