Revise "Profile" concept to "Presentation", per F2F consensus

[msporny]

This PR changes the concept of a "Profile" to a "Presentation" based on the consensus achieved at the VCWG F2F in San Francisco 2018. Fixes #163.

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[stonematt]

LGTM

[David-Chadwick]

LGTM

[msporny]

Blocking on @nage's review. Going to pull this in if we don't hear from him before the next telecon.

[msporny]

This PR is blocking a large series of editorial changes I need to make as a result of @vieillevigne's review comments (and there are a LOT of them... and I only have today and tomorrow to work on them).

I'm pulling it in since 1) there have been no objections so far (and lots of support), 2) because it just executes on what the VCWG agreed to in the F2F meeting, and 3) because @nage is very busy and will probably not have time to review for another two weeks.

I'm scheduling time to talk w/ Evernym and Sovrin Foundation to get down to what they need out of presentations in the next two weeks.

If there are objections after the fact, we'll deal with those in a separate PR.