Add build-client target to build also GLClient in dh_build. Fixes #998.

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?
Remove client/Makefile.

Client is using `grunt` as utility for building the project, I don't see the
point in using also `automake`. Furthermore, this file seems to be unused.
Change setup.py to fetch built client in ../client/build.

* Fix setup.py so that GLCLIENT_PATH results a constant, with an updated path.
* FIx debian/rules accordingly.
Do not add "grunt-cli" to client/package.json.

It is not really a requirement, but it is needed in order to build the client.
I am solving this issue by:
- Installing globally on a travis machine,
- Installling locally for building the debian package,
- Let the Globaleaks developers do whatevers they want on their system (either
  have it globally or locally, but not forcing on any of the two).
run the local version of grunt, even if that raises a warning.
Maybe there are better ways to do this, but as of now I don't see any 'grunt'
binary shipped by debian, and we really must build glclient without beeing
root..