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 think this should be `_string.length() - _pos`, but perhaps it would be better to work with iterators in this function (_pos would also be an iterator, and we would have another iterator _end, but will not have _string):
```
auto nl = find(_pos, _end, '\n');
copy(_pos, nl, back_inserter(_tagString));
return nl == _end ? _end : ++nl;
```