Use backend/ and client/ directories in travis scripts.

* Fix .travis.yml with the new directory structure.
* Globaleaks development mode sources from client/
* Fix path names in globaleaks.db.datainit.
* Remove beckend and client's travis.yml.
Add build-client target to build also GLClient in dh_build.

This is a quick and dirty fix for building client dependencies with npm, build
glclient, and make the backend aware of it. Specifically, the following must be
changed sooner or later:
* Do we really need to run that npm install in the build target?
* Can we update setup.py to avoid the symbolic link over there?