Move httpclient dependency into another gem

[krissi]

Hi,

to be able to handle custom http-servers I did a little refactoring.

This PR extracts the dependency on httpclient into another, small gem which may be skipped.

Using Faraday as mentioned in #15 might be better, but this works for now as an interim solution until faraday is usable.

It also opens the gates for a experimental faraday adapter ;)

In hope to hear other opinions, krissi