Started working on functionality for adding filter dropdown for taxonomies
* Adds a method for adding previously registered taxonomies
* Adds possibility to add taxonomies as filters for admin screens. Fixes #4
Hides the taxonomy filter dropdowns if they are empty
* Adds a $admin_filter argument for add_taxonomy()
* Made some refactoring and code commenting
Merge branch 'dev/filterable-by-taxonomy' into development:
Implements methods and arguments to add taxonomies as filters on the post type admin list.
README modifications, adds info on the new taxonomy functionalities
Merge branch 'development'

Conflicts:
	README.md
Upgrades WGMetaBox to v0.5.5