Basic json support

Really basic support for JSON.

 - A `RemoteTable::Json` module, which uses `JSON.parse` to extract json data
 - A new initializer arg `root_node` which specifies the JSON root node
 - Some inline docs for the arg change above
 - Tests and test data for JSON, both with a root node and without
 - Added `.travis.yml` for automated tests (currently not passing!)
 - No changes to the gemspec, as yet

This work is in reference to https://github.com/seamusabshere/remote_table/issues/18