Added support for TravisCI

Add continuous integration with TravisCI. For now, in the absence of unit tests, it is only validated if vzlogger does actually build. Both gcc and clang are configured.

One note on libsml: the "official repository" linked from http://wiki.volkszaehler.org/software/controller/vzlogger/installation_cpp-version doesn't compile (examples are broken). The travis build script uses https://github.com/TheCount/libsml/tree/develop instead.

@TheCount: perhaps you could check if the wiki needs updating?