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
I actually started to like "using namespace std;" in implementation files, which would (perhaps in addition to a typedef for std::string::const_iterator) make this file more concise such that while reading you are not distracted by all the verbose type names which are basically all the same, but:
How exactly is `back_inserted` accessed here? I don't see a way how this would work without a "std::" prefix, but it does.