Added .travis.yml file for travis-ci support

This travis-ci configuration file add support for building and testing both C++ and Python bindings tests, using both gcc and clang in Debug and Release builds. 
To complete the support the orocos/orocos_kinematics_dynamics repository should be enabled by the owner on https://travis-ci.org/ . 
Is possible to view an example build in https://travis-ci.org/traversaro/orocos_kinematics_dynamics/builds/21184828