Changes for ldp:memberObject ISSUE-72

Wanted to call attention to the changes I made [1] where I added a new
section 5.2.10.  Specifically the case where I need to handle the "default"
value of ldp:membershipObject.  One would naturally think of something like
<rdf:about> but this appears to only be part of RDF/XML [2].  So I created
an individual called <ldp:MemberSubject> that can be used to specify when
the minted URL on POST create is the same one to be used as the membership
object.

[1] - https://dvcs.w3.org/hg/ldpwg/raw-file/default/ldp.html#ldpc-5_2_10
[2] - http://www.w3.org/TR/rdf-syntax-grammar/

- Steve Speicher