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[RFC] Proposed change to directive location introspection

This proposes a change to how we represent the ability to validate the locations of directives via introspection.

Specifically, this deprecates `onField`, `onFragment`, and `onOperation` in favor of `locations` which is a list of `__DirectiveLocation`.

**Rationale:**

This allows for a more fine-grained validation of directive placement, now you can assert that a directive is allowed on queries but not mutations, or allowed on fragment definitions but not on fragment spreads.

Also, this makes expanding the locations a directive is allowed to be placed easier to do, as future expansions will not affect the introspection API. This should be considered a prereq to #265.

Finally, this is a prereq to a forthcoming RFC to add directives to the type schema language, one of the last missing pieces to represent a full schema using this language. Currently considering something like:

```
directive @skip(if: Boolean) on FIELD, FRAGMENT_SPREAD, INLINE_FRAGMENT
```

**Drawbacks:**

Any change to the introspection API is a challenge. Especially so for graphql-js which is used as both client tools via Graph*i*QL and as a node.js server.

To account for this, I've left the existing fields as deprecated, and continued to support these deprecated fields in `buildClientSchema`, which is used by Graph*i*QL. While graphql-js will likely continue to expose these deprecated fields for some time to come, the spec itself should not include these fields if this change is reflected.

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