#194: fixed bug with preserving category name in posts after renaming

This shoud fix #194 .

Bug: each post object stores all its categories. In the past, only category's id was stored. But right now, full category object is stored (both id, name and slug). It's probably done for performance reasons, despite there can be only a small amount of categories, as all of them are available through top menu.

My fix: I've added an update call after each category change. This call either updates category object in each post, or pulls it (if category was removed)