CMake: changed the library name from xmpp_space_control to xmppsc-client

xmppsc-client will be the future (package) name for this library.
CMake: added installation part, needed for packaging
CMake: Debian packaging

That's all that is possible with CPack and the Debian builder,
as a CMake project cannot be split into various Debian packages with different
settings (such as dependencies). This means that we cannot create lib and dev
packages with CPack.
CMake: use equal casing for all directives
Merge pull request #7 from penguineer/lib
Part solutions for #2 and #3.