Detail IRI template expansion in vocabulary and spec

- Add variable representations to Core Vocabulary.
- Append regular template example before free text.
- When introducing the concept of template variables, it's more instructive to show the regular case first. Also, this will make the explanation of representations more clear.
- Explain variable representations.
- Add representation examples.
- Warn about ExplicitRepresentation versus Turtle.

This addresses #30
Minor edits to the variableRepresentation section

- Removed second example as they only differed in the property used
- Add variableRepresentation to the example
- Converted the description of BasicRepresentation and ExplicitRepresentation from a definition list to a simple paragraph to be consistent with the rest of the document
- Moved warning up
- Added an exemplary IRI Template to the example comparing the two representation formats
- Add variableRepresentation, BasicRepresentation, and ExplicitRepresentation to the context and minor tweaks to their description.
- Make representation list consistent.
- Remove the domain of hydra:variableRepresentation

This closes #30.