Update incorrect docstring
Modify xml-test for root_prefix testing
Add root_prefix option

This adds a root_prefix search_option to search_prefix. It can be used
to limit the prefixes returned to those within root_prefix without
modifying the 'match' variable, ie if a prefix matched the search query
or not.

Fixes #206
Rename root_prefix to parent_prefix

It's really not a root prefix but rather a parent. This option will
influence not just where we start but also the number of indent levels
we get.
Fix parent_prefix!

This is almost the final fix. Now parent_prefix is the id of a prefix
from which we will base the search. Only prefixes that are one indent
level below and inside the parent_prefix are included.

A new exception is thrown if the prefix id specified does not exist. We
need to set a good error number for that and handle it in pynipap (and
jnipap).

Part of #206