Add composer.json - improvements

This PR builds on @Tarendai's work in #6 to add a valid, functional composer.json file to Mercator.

Changes I've made:

+   Fixed the description to use Mercator's description
+   Replaced two spaces with one tab character
+   Set the type to "wordpress-muplugin", so it gets installed to your mu-plugins folder
+   Added a comma that was missing