Cmake, travis, and others

[NikolausDemmel]

This is supposed to accompany https://github.com/JakobEngel/dso/pull/109

• Add travis build check (for now against my fork, until https://github.com/JakobEngel/dso/pull/109 is merged)
• Add more ways to find DSO. If it is built in catkin workspace, it is picked up automatically. If not, you can still set DSO_PATH manually.
• compiler flags updated to match https://github.com/JakobEngel/dso/pull/109
• updated package.xml