Setup continuous build with travis-ci

Hey guys, I'm Huy from the flickr-vision team.  This PR turns on a continuous build system for this project. All the builds will occur within fresh VMs hosted by the folks at travis-ci.org

The main highlight of this PR is the addition of the `.travis.yml` file which configures how this project should be built and tested. It will first install all the required dependencies for this project within an Ubuntu 12.04 container. Then it will run `make && make test` . 

It does not yet run `make runtest` however since the current setup requires a machine with a GPU for the tests to run correctly. I see that there's already a PR out for setting up CPU only tests: https://github.com/BVLC/caffe/pull/561. 

When that PR gets merged, a good sanity check would be to uncomment the `make runtest` line to verify that all the tests pass in travis-ci.

#### Other notes about the build workflow

The builds will run every time there is an update to the master or dev branches. When a build is finished, travis-ci.org will send a notification via email and via irc chat to the #caffe channel on freenode.

Here's a recent build from my own forked repo: https://travis-ci.org/huyng/caffe/jobs/29659970

If it looks good for merging, there are a few items we'll need to change so that the main BLVC/caffe repo has all the correct permissions and settings with travis-ci.org:

- [ ] add appropriate email recipients
- [ ] setup travis-ci account for BLVC and associate caffe 
- [ ] change README so that it points to the correct build status image