Migrate to cobra CLI system

Refactored the CLI commands to use [cobra](https://github.com/spf13/cobra). Nothing outside of the `main` package (the CLI entry point/commands) is changed by this PR.

Addresses #111

* Now subcommands automatically get a nice help command when running `ipfs help <cmd>`
* Both long and short flags can be used, e.g. `ipfs add -r` or `ipfs add --recursive`
* Flags can now be defined as either persistent or local. For example the `--config` flag is persistent and can be accessed by any subcommand
(also makes it really easy to break out subcommands into independent packages that can then be used by other tools (say `ipfs name` and `ipns` are the same cmd)