Adapters can have unsupported capabilities.

```js
import { extend } from "oasis/util";
import IframeAdapter from "oasis/iframe_adapter";

var MyRestrictedIframeAdapter = extend(IframeAdapter),
    myRestrictedIframeAdapter = new MyRestrictedIframeAdapter();

myRestrictedIframeAdapter.addUnsupportedCapability('someCapability');

// This sandbox will have 'a', 'b' and 'c' capabilities, but not
// 'someCapability'
oasis.createSandbox({
  adapter: myRestrictedIframeAdapter,
  capabilities: ['a', 'b', 'c', 'someCapability']
  /* ... */
});
```
Tolerate duplicate capabilities.
Merge pull request #67 from tildeio/adapter-unsupported-capabilities

Adapters can have unsupported capabilities.