reorganize developer documentation pages
fix issue #3: non-standard rId raises exception

* add try/except block to _Relationship._num to apply numeric value of
9999 to non-standard rIds. This results in those relationships being
sorted to an arbitrary position at the end of the list, behind
relationships with conformant rId values.

* comment out _reltype_ordering assignment in Presentation._load to
prevent presentation rIds from being renumbered, as renumbering was
causing inconsistency between slide master rId stored in
presentation.xml <p:sldMasterIdLst>

* factor __rewrite_sldIdLst() out to a helper method to clarify intent
in Presentation._blob

Full fix would require adding element rewriting code for all four
relationship lists in presentation.xml in Presentation._blob. That
effort is postponed until support is added for manipulating
slideMaster, notesMaster, and/or handoutMaster parts.