remove check on login
Merge pull request #793 from dotcloud/774-remove_login_check_on_pull-fix

- Registry: remove login check on pull
api_client: added APIClient type, with the first function

I'm including ListContainers here, so we can discuss some of its design
and prepare for future methods.
app: rename ApiContainers to ApiContainer and ApiImages to ApiImage
api_client: add queryString function

This function replaces the queryString method in the
ListContainersOptions struct. It uses reflect to "marshal" a struct or
struct pointer into a query string.
api_client: small nitpick
client: new package

Moving api_client.go and api_client_test.go from the
github.com/dotcloud/docker path.
client: import method "call" from the DockerCli type

I've renamed it to "do", and changed some of its behavior.
client: include version in the URL
client: added InspectContainer method
client: add some documentation