Revised name, require composer/installers for custom installation locations

[jrfoell]

I renamed the composer package name to webdevstudios/cpt_core to be more consistent with the git repository name and what's currently in https://github.com/WebDevStudios/Taxonomy_Core.

I could go either way with it if we want this to be installed as a plugin with the more normal dash-slug format: wp-content/plugins/cpt-core

If we go with dashes https://github.com/WebDevStudios/Taxonomy_Core should be updated as well.