After builing Freeswitch this makes actually run it easy by offering
an installer script.

* Changes to configure.ac to detect Debian8/CentOS 7 and create installer script
* Changes to build/Makefile.am to display the option
* Added init directory with templates and other files
* Deleted outdated build/freeswitch.service and tmpfiles.conf
* Unit file based on the latest systemd service file FS-8194
* Won't interfere with other platforms (afaik)
* Supports ./configure --prefix=
* Detects if Debian8/CentOS7 use systemd
* Can be used with a user with sudo permissions