Support for adding YAML defined policies to the catalog.

Includes support for loading from OSGi bundles.

The format is not in CAMP spec, just adhoc yaml, therefore feedback on the syntax is welcomed.
Sample yaml:

```yaml
name: Demo Policy
brooklyn.catalog:
  id: demo_policy
  version: 0.1.0
  libraries:
  - url: file://<path>/mms-sample-policies-0.1.0-SNAPSHOT.jar
policy: io.cloudsoft.ibm.mms.amp.policies.CompliancePolicy
```

If configuration in catalog is required (can be overriden when referenced in the app yaml):

```yaml
name: Demo Policy
brooklyn.catalog:
  id: demo_policy
  version: 0.1.0
  libraries:
  - url: file:///<path>/mms-sample-policies-0.1.0-SNAPSHOT.jar
policy:
  type: io.cloudsoft.ibm.mms.amp.policies.CompliancePolicy
  brooklyn.config:
    period: 20s
```

Analogous to config & catalog, perhaps I should prefix it with "brooklyn.".
I'd use `checkNotNull(policyConfig.get("type"), "policy type")`, with an `import static guava.Preconditions.checkNotNull`.