Scjson support

PR ready.

Assigned #230 to Jake because there is a bug on scion-core. Otherwise that test will pass.

Currently 156 tests are passing, 53 from W3C.
Closing 8 issues: 5 errors, 3 fails.

Closes #221
Closes #222
Closes #223
Closes #224
Closes #225
Closes #226 
Closes #227 
Closes #228 

Here, you should parse the `js` string with esprima, and traverse the AST, searching for nodes of type `AssignmentExpression`. For each node of type AssignmentExpression, compare `node.left.name` to values in `systemVariables`. You find it, then pass back the error.

There is an example AST for exactly this here:
http://esprima.org/doc/index.html#ast

`esprima.parse('var answer = 42;');` 

then the return value will be (JSON formatted):

```json
{
    type: 'Program',
    body: [
        {
            type: 'VariableDeclaration',
            declarations: [
                {
                    type: 'AssignmentExpression',
                    operator: =,
                    left: {
                        type: 'Identifier',
                        name: 'answer'
                    },
                    right: {
                        type: 'Literal',
                        value: 42
                    }
                }
            ]
        }
    ]
}
```