add react-in-jsx-scope rule

[glenjamin]

Hope this makes sense - I've included tests & docs as well.

See eslint/eslint#1905 for some additional context.

I'm planning to follow up with a variant on no-unused that knows about JSX using React.createElement.

Sidenote: should there be a test which verifies all rules are exported in index.js ?

[coveralls]

Coverage decreased (-1.92%) to 98.08% when pulling de17bfe on glenjamin:react-in-jsx-scope into 9c43de0 on yannickcr:master.

[glenjamin]

Whoops, missed a test case for nested scopes - will add one to bump the coverage back up.

[coveralls]

Coverage remained the same at 100.0% when pulling 5fec402 on glenjamin:react-in-jsx-scope into 9c43de0 on yannickcr:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 5fec402 on glenjamin:react-in-jsx-scope into 9c43de0 on yannickcr:master.

[yannickcr]

Thanks for contributing! Looks good to me.

should there be a test which verifies all rules are exported in index.js ?

There is not, but i should definitely add one.