testing: add Transport and ConditionalTransport types

These types provide fake implementation of http.RoundTripper. I'm
generalizing the code, because it's replicated through some cmd
subpackages/commands.
cmd/crane: use Transport and ConditionalTransport from testing pkg
cmd/tsuru: use Transport and ConditionalTransport from testing pkg
cmd/tsuru-base: use Transport and ConditionalTransport from testing pkg
testing: add Headers field to fake RoundTripper implementation
cmd: use Transport and ConditionalTransport from testing pkg
cmd/tsuru-admin: add tokenGen command

This command will be used to generate user tokens.

Related to #389.
cmd/tsuru-admin: register token-gen command

Closes #389.
cmd/tsuru-admin: version 0.1.1