Category initializers for MapStore and MapManager

* mxp.plugins.CategoriesInitializer for MapManager
* gxp.plugins.CategoriesInitializer for MapStore

This plugins check if there are a list of categories in GeoStore and create if not present.

Wiki page here: https://github.com/geosolutions-it/MapStore/wiki/Categories-Initializer (for gxp)

Fixes #386.
Minor fix on categories initializer

Set default categories to `["TEMPLATE", "MAP", "MAPSTORECONFIG", "ADMINCONFIG"]` in both plugins.
Merge pull request #402 from alediator/386_categories_initializer_3

Category initializers for MapStore and MapManager