add Travis-CI integration

Travis-CI [1] is a continous integration cloud service. It is free for
open-source projects and tigthly integrated tiwh GitHub so there is really
no reason for us not use it. :)

For now we are going to do a total of 4 builds, mainly to test ffmpeg/libav
API breakage:

 * ffmpeg-stable, libass-stable
 * ffmpeg-git, libass-stable
 * libav-stable, libass-stable
 * libav-git, libass-stable

The compiler that is currently used is clang for two reasons:

 * running 8 build targets would be quite wasteful and take a long time
 * clang is less tested and used during development than gcc (especially on
   linux)

Currently Travis doesn't support OS X environments alongside Linux ones [2].
When it will, we will add a fifth build target to test OS X compilation
breakage.

README was moved to markdown to add the little build status image. I ran some
tests with my GitHub fork and couldn't get images to show up using ReStructured
Text.

 [1]: https://github.com/travis-ci/travis-ci
 [2]: travis-ci/travis-ci#216