add Assert helper class, intended to simplify code
use new Assert class for simplifying code
increase download count on item download
include download count in item description
update MySQL table
add a MySQL table for keeping ratings
implement basic rating
add a method to get the API version
support specifying a homepage URL

In the schema, a homepage URL for the item can now be specified.
Include it in the item description (if present).

This closes #4.
change user description

Always include a 'mail-md5' field. The 'mail' field now always holds
the mail address IF it is present.
add a small function to the Item class
fix Item::getId()

Retrieval of "special" version numbers wasn't working.
item description: simplify code with Item::getId() method
include item type in listing
fix two incorrect parameter assertions
add new DB for item types
change type attribute to integer
reflect variable item types in schema
add a small class for managing item types
adjust code to the new type system