Refactor HTTP interface

[brenocq]

Description

This pull-request is related to the issue #14. The goal is to refator the atta::io::http. Previouslly, it was causing problems to build to Windows/MacOS/Web. We decided to switch from cpprestsdk to curlpp, which was proven to be easier to build and use.

Current implementation can only do blocking request, but may be extended in the future to support non-blocking requests.

A JSON parser/dumper was also implemented to help dealing with JSON strings.

Http implementation

Now it is only possible to do GET, POST, and PUT requests.

Http::Response Http::get(const Http::Request& req);
Http::Response Http::post(const Http::Request& req);
Http::Response Http::put(const Http::Request& req);

The Request and Response definitions are:

struct Request     
{
    std::map<std::string, std::string> headers;
    std::map<std::string, std::string> params;
    std::string body;
};

struct Response     
{
    unsigned statusCode;
    std::map<std::string, std::string> headers;
    std::string body;
};

JSON implementation

The implementation is similar to Section helper implemented in #13. Examples of how to use the JSON parser/dumper can be found in the JSON unit test file.