Refactored docker library

Hi, 

I think my refactoring is ready for review.  

Biggest changes are:

* refactored RPC calls into own namespace

* added client abstraction to keep state of client and manage it's settings. 

* added tests, although to run test under namespace `docker.mocks` you have to have newest [http-kit-fake](https://github.com/d11wtq/http-kit-fake) "0.2.2", which includes important fix for stubbing `httpkit.client/request` function. It may take some time until owner of this library will publish his library on clojars. But meanwhile you can clone [my repo](https://github.com/tauho/http-kit-fake) and install it as localrepo with lein-localrepo plugin. And that's the reason why Travis is currently failing.

* updated docs, Readme and added api reference .

Timo