Adding Recurly User-Agent

@pierre

Recurly has a User-Agent format that we use to track who is using what
version of what libraries. This commit adds that to all requests.


Perhaps you can suggest a way this version number can be rendered dynamically or in a way that fits in with your build process. It would also be nice to add the java version at the end of the line here. For example. this is how we do it in the Ruby client:

https://github.com/recurly/recurly-client-ruby/blob/be27fbb8132b84537b333a7d8913753162e24bc0/lib/recurly/api.rb#L78