[added] isObservable(maybeStream)
[added] isObservable(maybeStream)

Summary:
Because JavaScript is a duck-typed language, it's common to check for the presence of a member you care about inline:

```
if (maybeStream.subscribe)
  maybeStream.subscribe()
```

TypeScript uses [type guards](https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards) for this purpose.  A type guard is a function that wraps that test, and tells TypeScript that any value that passes the test matches a given type.

This diff adds a type guard for Observable.

Reviewers: featherless, #material_motion, O2 Material Motion, O3 Material JavaScript platform reviewers

Reviewed By: featherless, #material_motion, O2 Material Motion

Tags: #material_motion

Differential Revision: http://codereview.cc/D2548

material-motion/material-motion-js,material-motion/material-motion-js,material-motion/material-motion-js