rels: add get_rel_of_type() method to RelsCollctn

No test coverage. Required by OpcPackage.main_document() method.
part: make blob optional on construction

If sub-class decides to keep track of part content, by for example
transforming it into an ElementTree element and converting it back to
XML on blob() call, there is no need for Part base class to keep a
duplicate copy of the original content.
part_fctry: change CustomPartClass contruct to load()

Use a classmethod constructor (.load()) for CustomPartClass rather than
constructing directly. This allows custom part class to do some
pre-processing before actual construction, and keeping its __init__()
method clean, containing only simple assignments to state vars.
pkg: add .main_document property

Locates the main document part, e.g. word/document.xml for a Word
package, ppt/presentation.xml for a PowerPoint document.
doc: add pull at own risk warning for spike branch