Include editorconfig core when installing

The install script didn't include installing the core library, leaving the plugin broken; this addition installs the entire package.

Side note: my gedit seems to use my home directory as its working directory, so when shared.py appends `abspath('../editorconfig-core-py/')` to `sys.path`, it actually added `'/home/editorconfig-core-py'`. I didn't remove this as it seems to be harmless, but you may want to take a look at this as well.