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
The two reset functions are only used once each. Might it be possible to remove the global state they reset and have a dedicated class whose lifetime is bound to the scope where the two reset functions are currently called?