Identify client SDK + version in request headers

[ire-and-curses]

As described in stellar/go#725, it would be really helpful if this SDK sent two simple headers with each request, identifying the SDK and version:

two headers are sent: client name (X-Client-Name) and version (X-Client-Version)

This will help maintainers of public Horizon instances understand where their traffic is coming from, and allow them to assess the impact of upgrading their Horizon version (for example they might delay upgrading to a version with breaking changes not handled by a client SDK with significant traffic to the server).