Support symbols

@bfulton @tlunter 

This pull is a work in progress, but the meaty parts are done. It adds symbolic key support to `docker-api` (a feature asked for time and time again). It does so based on the assumption that query parameters accept `lowerCamelCase`, and JSON body parameters accept `UpperCamelCase`. As far as I can tell, this is only untrue for one API method, `/containers/#{id}/top`, which accepts its argument in `snake_case`. Let me know what you think, as my solution clearly isn't the DRYest.

Also, if this were to be merged, bump to `2.0.0`, or `1.11.0`?

Remaining work:
* Add symbol key support to `Docker::Event`?
* Update the README
* Version bump