Provide HTTP exposition

[korfuri]

Currently, the client library does provide protobuf and text representation of a collection of metrics, but no bindings to a HTTP server to expose it at /metrics. We should provide bindings for various HTTP server implementations. Here are some ideas:

• cpp-netlib
• Poco::Net::HTTPServer
• GNU's libmicrohttpd (it's in C, we'll probably need to define C bindings for that)
• Proxygen
• Casablanca

There are many more and I'm not quite sure which ones are popular. I will probably only provide one or two and document the integration process so others can contribute their own integration.

[korfuri]

Gnu libmicrohttpd integration was added in 5904d6f3233a3819364409e996ecddb0d196b89d. I plan to integrate with at least one other system and factor out useful parts to make future integrations easier, and then I'll consider this done.

[agnat]

@zaphoyd/websocketpp integration would be nice. I'll look into it.