Add ability to pass JSON object as attribute value.

With changes introduced in this pull request it is possible
1) pass JSON object as attribute value
2) attribute value can contain braces

Also I've replaced eval with JSON.parse, I think it is save to assume that attribute contains static value that we just need to parse, not to evaluate.

I would like to add some tests for these cases, but as of now there is no tests infrastructure at all. Any plans with regards to test?