Add check capability endpoint.

+ Move the capability check inside `validate_call` to the
`check_capability` method.
REST API: Return an array of capabilities.

The endpoint can be called passing a single capability or an array of
capabilities.
In the first case, It returns a boolean indicating if the current user
has that capability.
In the second case, an array with the capabilities as keys and
current_user_can( key ) as values.
Merge pull request #883 from Automattic/add-check-capability-endpoint

Add check capability endpoint.