Draft algorithms specification

Anssi, Dominik, et al,

I have created a first draft of the algorithms [1] to specify the behavior
of the Presentation API as defined by the current draft of the
specification [2].

I believe the behavior is consistent with the current state of discussion
on the mailing list; however, I have annotated the PR with several open
issues that may deserve further discussion.  If the editors and WG members
agree with the direction and content of this PR I am happy to iterate on it
further.

I am newly acquainted with the indented-list style of pseudocode used by
the W3C so if anything is unclear please provide feedback here, or in
conversation on the PR.

I had some difficulty using the current Makefile to generate cross
references.  It seems like the content of the xrefs repo in the CG was not
fully ported to the w3c repo - anolis gives an error when trying to process
cross references.  Perhaps that can be fixed separately and I can rebase my
changes so I don't include a hacked up Makefile in my PR.

Cheers and Happy Holidays,
Mark

[1] https://github.com/w3c/presentation-api/pull/37
[2] http://w3c.github.io/presentation-api/