HttpSig update (#235)

* add the fetching of the ACR to the diagrams

* Added missing ":" to Turtle Example

Fixed prefix declaration in the Turtle doc preamble.

```
@Prefix security <https://w3id.org/security#> .
```
changed to
```
@Prefix security: <https://w3id.org/security#> .
```

* feedback from implementation https://github.com/co-operating-systems/Reactive-SoLiD/blob/working/test/rfcKey.1.ttl

* add rdf: namespace for JSON. Formatting

* update URL links to "Signing HTTP Messages" spec

* link to discussion on which headers to sign

* Change Sequence Diagram to graffle, add source

* increase resolution to 200dpi for retina displays

* symbolic link from old HttpSignature.md to new one

* clarify sequence diagram, and its description

* update Sequence diagram

* smaller loops

* move to svg for pictures

* trying to get WebID svg to show TimBL pic

* replace tiff with jpg of timbl

* link to WebID social svg

* last attempt to get timbl image to work

* note on bug in OmniGraffle

* add csarven's html version of httpsig

---------

Co-authored-by: OpenLink Software <github@openlinksw.com>

Author: bblfish