Script to find missing samples

This is a quick Ruby script designed to find missing samples anywhere in the `docs/` directory. It currently reports a line for each (service, sample, language) tuple that doesn't contain at least one nonempty line in the appropriate `.rst` file, along with the total counts.

To run it, use:

```bash
vagrant ssh -c 'ruby /vagrant_data/sample_completion.rb'
```

Of course the `vagrant ssh` bit isn't strictly necessary if you've got a recent, sane Ruby install on your local box.

The output format looks like:

```
(...)
orchestration        / delete_stack                   / csharp      : missing
orchestration        / delete_stack                   / java        : missing
orchestration        / delete_stack                   / javascript  : missing
orchestration        / delete_stack                   / php         : missing
orchestration        / delete_stack                   / python      : missing
orchestration        / create_stack                   / csharp      : missing
orchestration        / create_stack                   / java        : missing
orchestration        / create_stack                   / javascript  : missing
orchestration        / create_stack                   / php         : missing
orchestration        / create_stack                   / python      : missing
orchestration        / authentication                 / csharp      : missing
orchestration        / authentication                 / java        : missing
orchestration        / authentication                 / javascript  : missing
orchestration        / authentication                 / php         : missing
orchestration        / authentication                 / python      : missing
summary: 402 complete / 438 missing / 840 total
```

If you want, you can use `grep` and `wc` to see only what's left for a particular language or service:

```bash
smash@tiger ~/writings/developer.rackspace.com (find-holes=) $ ruby sample_completion.rb | grep python
auto-scaling         / create_scaling_group           / python      : missing
auto-scaling         / create_scaling_policy          / python      : missing
auto-scaling         / create_scaling_webhook         / python      : missing
auto-scaling         / delete_scaling_group           / python      : missing
auto-scaling         / delete_scaling_policy          / python      : missing
auto-scaling         / delete_scaling_webhook         / python      : missing
auto-scaling         / execute_scaling_policy         / python      : missing
# (...)
smash@tiger ~/writings/developer.rackspace.com (find-holes=) $ ruby sample_completion.rb | grep python | wc -l
      46
```

Hopefully, this should give us a source of truth for tracking progress on #119, and make sure that no samples fall through the cracks, especially if we add samples to a service after someone has already "completed" that service for another SDK.