Travis CI setup

Hi,

Not sure if this of interest, but if you are here it is...

This pull request adds a .travis.yml to configure async to be built by Travis. It also adds a link to the readme page showing the build status.

Here is an example of what it looks like:

https://travis-ci.org/kimptoc/async

Along with this pull request, you (caolan) will need to setup your Travis account (preferably first) - see steps 1 and 2 in this doc:

http://about.travis-ci.org/docs/user/getting-started/

Regards,
Chris