Setup Continuous Integration on Travis CI

Travis CI works coupled with github. Just registering public projects allow one to run continuous integration on Linux and Mac Boxes. 

Using this pull request, registering dealii on Travis CI, will make all pull requests tested on Linux (and Mac) on a public server. See 

https://travis-ci.org/luca-heltai/dealii/builds

for an example. 
I left it there, because I don't know if it works on mac. So far, they have not enabled mac compilation on my project yet... 

When our project becomes a "git featured" project, they will enable also mac compilations. Then we'll know if mac works or not. I'm leaving it there just to have catch on what to write for mac compilations...