added .travis.yml

* Enables Travis continuous integration for github
* Currently only {with,without} fftw and {with,without} mpi is tested for gcc and clang
* more build environments can easily be added or copied from Jenkins
* main advantage: pull request will have be checked automatically by Travis (see the green check mark below) and
https://travis-ci.org/ottxor/espresso
https://travis-ci.org/ottxor/espresso/pull_requests
without the need to run Jenkins.