Exporting Natspec documentation to JSON

- Adding the possibility to export Natspec documentation from functions to a JSON file
- Adding tests for the Natspec documentation exporting
- JSON exporting, both for Natspec and ABI interface now uses jsoncpp library instead of forming the JSON string by hand.
    + Note: Until Sven's #532 PR makes it into develop, this should probably not be merged. Currently including JSONRPC-CPP as a libsolidity dependency just so we can get jsoncpp. With Sven's PR we will be able to include the proper dependency.
- Better Test error reporting for the JSON ABI interface
Would it perhaps be a good idea to have a completely separate class which is supplied with an AST node?