Fixing item creation, simplifying item dict cleanup

Fixing bib item retrieval, improving get_subset

Fixing a User ID bug

Write API methods are now side-effect free w/r/t retrieved items

Adding check_items() method

Adding method to check for server-side item updates

Adding Sphinx documentation

Added a method to retrieve arbitrary subsets of items.

Cleaned up code and comments

Tidying documentation, docstrings, method names

Adding Write API methods:

Create item(s)
Update item
Delete item

Using defaultdicts for items, collections and groups.