First version of Hydra

While the vocabulary is complete, nothing has been properly documented yet.
Minor fixes
Update homepage and references
Update to ReSpec 3.1.51
Update to latest jsonld.js (had to patch it to expose window.jsonld)
Update Introduction, Conformance, and Hydra at a Glance

The vocabulary itself has not been updated yet.
Update vocabulary definition
Describe how clients can discover the API docu and the main entry point
Add identifier to API documentation in example
Add first cut of "Adding Affordances to Representations"

Hide "Using Hydra Descriptions as JSON" and "The Hydra Core Vocabulary in JSON-LD" and add issue markers to the class and property definitions.
Describe links and add placeholder for section "Advanced Concepts"

This completes the first version of the "Adding Affordances to Representations" section
Add support for comments in examples
Add first version of "Documenting a Web API"
Rename "Accessing a Hydra-powered Web API" to "Discovering a Hydra-powered Web API"
Update vocabulary definition and include it in the spec
Remove issue markers, the vocabulary is now up to date
Update README
Fix links in README
Minor fixes in "Discovering a Hydra-powered Web API"
Add Collection/PagedCollection concepts to core context