Validate cloudlet registry YAML

This commit adds a couple pieces of functionality:

1. Rather than accepting whatever the GitHub page gives us, use
`yaml.safe_load()` and `yaml.safe_dump()` to make sure that we're
storing valid YAML in the registry.

2. In the event that we try to load registry data and receive invalid
YAML, throw an informative error message and clear the registry cache.
Merge pull request #158 from huit/bugfix/152_validate-yaml-before-storing

Validate cloudlet registry YAML
Merge branch 'master' into 88-cli-bootstrap