Add support for completion scripts

###### Notes:

- The script can be printed via the `--completion-script` flag. This flag is available only to the root command.
- Dynamic generation of the completions, as they can change with plugins or new version of a tool, was inspected but the RubyGems overhead makes the process to0 slow (around 0.5s).
- At the moment only support for Z shell is provided. Providing it for other shells should be reasonably easy now that the is an implementation but my shell-fu is very limited.
- This approach is very fast.
- Option completions are triggered only when the `-` character is typed, by zsh.

###### Possible future enhancements:

- Some commands might still benefit from suggestions generated by the command line tool, for example `pod search` or `pod lib lint`.

###### Gif

![2014-05-04 20_29_51](https://cloud.githubusercontent.com/assets/1048705/2873714/2aaa8c4e-d3ba-11e3-965f-6d6fd3ea524b.gif)


/c @alloy @Keithbsmiley @AliSoftware 

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Closes #11