Add dataJson option in geographyConfig

This PR allows the use `dataJson` in `geographyConfig` to specify a custom topojson file as a string, instead of having to use `dataUrl` and link to an external file. This is especially useful in generating standalone html files with no dependencies.
dataJson should be valid JSON and not a string.
Merge pull request #121 from ramnathv/master

Add dataJson option in geographyConfig