Restructure kite/cmd directory for better readability.

All subcommands are in kite/kitectl/command/ directory. Main application
is in kite/kitectl/ directory with the name main.go, like it should be.
Rename 'cmd' package name to 'command'
kitectl: Find the cli package and pass the tests.
kitectl: Use github.com/mitchellh/cli

mitchellh/cli is more tested and more extendable CLI helper. Since
kloudctl is already using it, we can safely use it for kitectl.

Apart from changing the CLI package, I also added help/usage texts and
more error checks.
kitectl: Reflect the changes that are mentioned in the PR.

- Do not stutter by appending `Command` to every subcommand structs.
- Remove version command since the new CLI package already provides this
  functionality.
- Add a default.go file that shares the default objects such as
  Default CLI UI and Default Kite Client.
- Remove population of the commands in main.go. Move this construction into the related subcommands.
- Test kitectl in Makefile.
kitectl: Oops, revert back the ulimit change in Makefile.
Merge pull request #55 from igungor/kitectl

Rename kite binary to kitectl and restructure the kite/cmd directory.